I was travelling to Seattle from London a few months back, and anticipated the same sort of data costs. I worked around it quite easily by using MaemoMapper, and using my WiFi at home to download and cache maps for the region. I did this for Google Maps, Google hybrid maps, and OpenStreetmap, giving me the ability to switch back and forth. If I remember correctly, I pulled down map zoom levels of 11 & 9 for the entire western third of Washington, levels 9 & 7 for the specific Everett - Federal Way section, and 9-7-5 from downtown out to the areas where I was going to be staying with relatives. I then added a similar detailed set data for the San Juan Islands, as I thought we might go whale watching. Downloading took a little bit of time for each set, but it didn't distract me much from the TV show I was watching at the time. Elapsed time was 1 hour or so. This was on my N810.

What this gives you is *not* routing, it gives you maps. If you want routing, the data cost of going online to compute a route and then use it offline with the cached maps should be low. Since there are no voice directions, you'll have to depend on your co-pilot to read from the N900. Unless your co-pilot has higher gadget tolerance than mine, swapping driving duties might be sensible. For holidaying, I find maps from OpenStreetmap to be significantly nicer than the "just roads" focus of TomTom/Naviteq maps as the wiki-like nature of things positively encourages people to list parks, walking paths, etc. http://www.openstreetmap.org/?lat=51...layers=B000FTF and http://www.openstreetmap.org/?lat=-4...layers=B000FTF are two examples.

There's a good set of quick bullets on MaemoMapper at http://wiki.maemo.org/Navigation_Tools.