I recently got contacted by a motorcycle traveller who has similar problems to the ones I describe on the bottom half of this post:
http://www.horizonsunlimited.com/tst...lex/004281.php
That was 2009, but the same stuff is still ongoing in 2012.
In short, having left Tehran, you're heading east because you want to check out the beautiful Kavir desert. You reach Semnan and then take the "36" road because that's the quickest way to the 81 heading south, cutting through the desert via Mo'Alleman.
Bad idea. You're stopped by the police and treated like a spy suspect. If you're lucky, you just waste a few hours. If you're unlucky (or stupid, as I was), you get to sign documents you can't read, hand over some biometric information and the cops take your bike for a spin. If you have the wrong passport and are unlucky... things can turn pretty hairy.
I'd rather suggest you approach the desert via Damghan. Yes, it's a ridiculous detour, but it keeps you away from the area east of the new Semnan airport which Iranian police appear to be nervous about.
The tricky area:
https://toolserver.org/~osm/locale/e...4415&layers=BT
I suggest you stay on main roads and avoid any shortcuts, no matter how well signposted, around that area.
Iran is a beautiful country, the Iranian people are fantastic (cultured, smart, hospitable) but I'll make an exception for the police. The overwhelming majority of Iranian cops I dealt with back in 2009 were absolute jerks.