I'm trying out several mobile mapping solutions. None of them are really great, and none of them (apparently) are 'hackable'. It's very easy to get location information from GPS, network, or the user. What's not easy is getting and using maps. Projects like OpenStreetMap are trying to create shared, open maps for users. But this is a long way off, and will never have the support and infrastructure that the commercial map providers have.
ThoughtFix has a plea to Nokia to provide a mapping application for their Nokia 770, and points out the (hereforeto unbeknownst to me) Route66 Mobile which can run on my Nokia 6600. I already have a BlueLogger GPS, so the $300 price-tag including GPS unit is a bit steep, though the package does include lots of goodies (RS-MMC memory card, card reader, GPS unit car charger, phone car mount).
What would be great, however, would be able to use the map files included with Route66, or Delorme Handheld StreetAtlas, or any other maps meant for a mobile platform (small resolution, small file size). Then developers could provide additional solutions and add-ons via their favorite programming medium. The high-cost of map delivery really precludes innovative, market-risky application development.