Hi All
As I couldn't find much detailed info on this crossing before we did it, I will try and give some details of the Libyan/Egyptian border crossing. We entered Libya from Tunisia and the details on the Sahara Overland site were adequate in getting us through that. Thank you, Chris.

EXITING LIBYA AT MUSAAD (aka AHSAAD)
About 100km from Tobruk, the road forks with English signposts for the first time! Take the right hand fork, signed Musaad (Ahsaad on the maps). Drive through Musaad (about 130km from Tobruk). Just before you get out of town, look out for a building on the right-hand side of the road with flags flying and several cars parked outside. You MUST stop here to return your Libyan plates and get back 100 Libyan Dinar. When you do this, give them the group of 3 papers that were issued with the plates on entry. They take the top 2 copies and you keep the bottom, larger copy. Nothing is stamped, but it was all accepted by customs at the border. Note that if you miss this, you may end up having to return after already being stamped out of Libya, which is what happened to us and another vehicle we saw.
Outside of the town of Musaad, you have to drive along 2 stretches of ragged road (detour off the main road, which seems to be in the process of being repaired) before you reach a checkpoint. Stop before this checkpoint and get your carnet stamped to export the car.
Go through the checkpoint and the immigration kiosks will be obvious ahead of you (choose any, but it's worth getting out and asking at one of the kiosks in order to avoid getting stuck behind a taxi of numerous people!). Get your passports stamped here and then go through the checkpoint where you'll need to produce evidence of your plates being returned (aforementioned slip of paper) and your passport. There is a second checkpoint after the 'Duty Free Shop on the right. Passports again.
Then you're through, into a short stretch of No-Man's Land before the Egyptian checkpoint (Sallum).

ENTERING EGYPT AT SALLUM
Go through the checkpoint and head for the hangars ahead. Park up and go into the first hangar on the left. Complete and Arrivals Form and get your passport stamped at the east end of the hangar. There are plenty of Port Police to help here. Immigration is then complete.
Now the car. What a performance. This part of the procedure took us 2/3 of the entire time it took us to get through Libyan and Egyptian borders and was, as Chris says, an arseache of note. We were assisted by a couple of really nice gents, who asked for nothing in return, which was completely unexpected. However, it's easier to be helped by these people (who rush you from place to place) if you have an idea of what the procedure is. Here goes:
Drive your car up one of the driveways on the right, looking for someone who'll wave you up. You'll be parked here for a while. The procedure is:
1. Get your chassis number validated against your carnet.
2. Change money either at the bank (only dollars/euros/pounds) or the money changers outside (Libyan Dinar). You need a total of 607 Gippo pounds (EP) to get you and your car into the country, so make sure you've got that before you go any further.
3. In the hangar, go through the corridor at the west end (opposite end to the immigration kiosks) and follow the passage right to the back to the man who deals with carnets (referred to Triptique). If you flash it around, most will understand where you need to go and can direct you. Carnet Man will complete and stamp the carnet and then ask you for 502 EP.
4. Go to the cashier to pay over your 502 EP (Carnet Man should direct you), get a receipt and take it back to Carnet Man, who will look at it, shuffle papers, give you the import section of your carnet (keeping the rest for the time being) and then instruct you on the next step.
5. Go back to the car to get a rubbing taken of the chassis number. Ours was done by a white uniformed man, but I've no idea what authority he represented. Make sure the rubbing is clear, because if it's not, it'll be rejected and it'll have to be done again. The rubbing appeared to be taken on a form expressly for this purpose and the number on the chassis itself was taken, not the plate on the top of the Land Rover engine.
6. Take the rubbing, the importation portion of your carnet (as issued in step 4), the carte gris (registration papers for the car) and your passport over to the building housing the Traffic police, which is further up the road on the right. (You should be allowed to drive here if the rubbing is acceptable.) Enter the building and look for the office on the left at the end, where there is a copying machine and an operator who will charge 25 EP to copy the visa page in your passport and the importation portion of the carnet. (I had copies of everything else.) He'll put all into 2 folders (not sure what order, but he knows and you can leave it at that).
7. Buy insurance (25 EP for one month) from the office next door to the Traffic Police.
8. Go back to the Traffic Police office with your insurance and all other papers collected along the way (including the relevant copies). Pay the Traffic Police 50 EP. You'll be issued with temporary license plates, which will be attached by a man outside for a further 5 EP. (You could do this yourself, but if it's late and you've been there ages, 5 EP is a small price to pay!)
9. Take the paperwork issued by the Traffic Police and copies of stuff made in step 6 back to Carnet Man. He will check everything, put it all in a folder (why???) and give you back your carnet.
10. Go back to the Traffic Police to collect your Egyptian license (credit card sized thing that you'll be asked for at all checkpoints in Egypt, as well as various tourist sites).

There is one last checkpoint before you're in Egypt proper. Have your passport and Gippo license ready. And you're through. Enjoy the pass down into Sallum town, which is pretty spectacular after a relatively flat coast road in Libya.

HANDY HINTS
Get the details page of your passport translated into Arabic (including the dates and numbers). This speeds up the process considerably as you won't need to spend ages pronouncing all your names, etc, slowly with increasing irritation! In Libya, very few people speak English and the border is no better. In spite of expectation, the Egyptian side of the border was equally
Arabic and our translated passports did help. Have several copies so you can hand them out.

COSTS
I tried to keep tabs on costs, but after 3 hours of shelling out money to various authorities, I lost track. We popped out the other end 607 EP poorer, which amounts to 84 Euros at the border exchange rate of 7.23/1EP.

Good luck
Bel