The Map application for Nokia's OS2008 (for N800 and N810) lets you download map data for a number of regions. The USA-West and USA-East regions are very large, though, and I have never been able to download them -- it always fails about half way through. I know others have dealt with the same problem.
This morning I got a reply from Wayfinder Customer Support:
Dear Sir, Thank you for contacting Wayfinder.
If the map download fails through the Internet Tablet, you can download the maps from this address: http://www.navicoretech.com/Consumer/Support/Downloads/tablet/en_GB/wfnavigator/
Best regards, Annette Customer Support Wayfinder
Instructions for installing the map data are on that site. It's still a slow download but at least you can use a download manager. If you're syndicated on Planet Maemo please echo this there.