While I personally would love handy maps on my iPod, I don't think this is practical. Maybe having the maps for certain areas would be possible.
Note, with a little elbow grease it's possible right now. Do a google search for "Yahoo directions ipod" (sans quotes) and, with a little modification you can tailor that to Rockbox. I've done it. It was nice on the one trip I did (esp. when I got off course) but I don't think I'd go through all the work again.