Hi Hugh, now you're talking!
AFAIK, there are two main cross-Am routes - east-west called the Trans Am Trail as Grant says - set up by a guy called Sam Correro:
http://www.transamtrail.com/
Russell Fisher did it on a XL650L, posted a website and I think it's linked off or on HU (but not on my site anymore as it says on p.199 of the current/expiring AMH).
Although ocean to ocean sounds nice, I get the impression that the eastern half is a bit of a drag. It's for this reason that I get the feeling the TAT has not really caught on with US riders, great idea though it is.
Much more impressive (tho' I have not done it either! just investigated it over the years - most recently last summer and probably again this year) is Can to Mex across the western states - actually various routes.
The Great Western Trail project (p.193 in AMH) is still incomplete but IMO promises a stonking ride though the deserty/Rocky mountain states - never a dull mile. A part-time US user of HU, Allen Naille, has been involved in AZ but as you'd expect, some NIMBYs dont want OHVs, especially up in Montana it seems.
Even then, I found in Montana there are enough legit fire roads etc right up to Eureka, even if it may require some tarmac links. On the GWT you'd want to ride into the summer from Mex or into the autumn from Can.
Some pushbikers did the GWT and wrote a roadbook/website. They happened to use many trails open to motor vehicles so their roadbook/web is useful to all (lost the link).
Some guys run also run own-bike, 2-week m/c tours from up the west coast states with hotel accom every night for $2500
http://www.mex2can.com/
I picked up a NatGeog Adv magazine last summer with a GWT feature (ID, OR, NV, CA). I can send you a pic of the article if you want.
A3-sized DeLorme states atlases would be a great help in route finding (also available as GPS downloads, I beleive).
With the west USA's landscapes and history and people, GWT would be a fantastic dirt ride.
Chris S
PS - just found this link below - they did the 'Adventure Cycling route (incl. link)
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Author of Sahara Overland II hardback edition and the Adventure Motorcycling Handbook - new edition shortly
http://www.sahara-overland.com and
http://www.adventure-motorcycling.com
[This message has been edited by Chris Scott (edited 29 January 2005).]