I've been been thinking about a similar trip, but this year it will only be from San Diego to Portland. I have a Garmin eTrex Vista Hcx and have maps for USA and Europe. I don't know what to do about points south. I'm considering using google maps and downloading critical points as waypoints.
I concur with Fred, handful of batteries and a charger. I think that solar panels will be impractical, as you'll probably be lucky to get a set of batteries charged while you're riding. I looked into hub generators and the one guy who knew a lot about them (sorry, I don't remember his name right now) said that they don't tolerate water well and they have to be rebuilt after a dip in a creek. He is going to use batteries and a charger.
Note on power usage of the the eTrex Vista Hcx (and probably all Garmins). The more track points stored, the more drain on the battery. The battery drain is double when you store the data to the microSD card. (Sounds weird, but there are indeed technical reasons for this.)
You're going to want to document your trip; lots of pictures, write in a journal. But you know this. If you synchronize your camera's time to the GPS everyday, (camera time will drift) you'll be able to geotag your photos. From there it is an easy step to create a digital map with digital pushpins showing where every photo was taken. Eventually you'll get to a place where someone has a computer and you can download your track and photos and then send them to some who can do this for you.
Personally, my strategy is to take a 1.5kg weight hit and bring a small computer. With all free (not pirated) software and a bit of glue code, I can save a day's track and pictures, then create a web page automatically in 5 minutes. I'm going to photograph my written journal and download that at the same time. When I get to a place with internet access, I'll upload my pages to my web site. for example
http://www.greggferry.com/travel/2009/05/06/ I'll be looking forward to following your progress!
Bon voyage,
-Gregg
ps, take a back up GPS, I've had two bounce off the mount, one was two-dimensionally transmorgified and the other is back for repairs.