Our city uses Localist which is a paid product, but it's customizable and works well. http://www.localist.com/. You can see the implementation here: http://whatweekly.com/calendar...

Hacking your navigation device will be HOT after the community develops three things.

• Passively user generated map data (not to be confused with actively generated map data from openstreetmap.org) (see blog entry comparing active vs. passive maps)
• Open source navigation routing algorithms to use the above map data.
• Devices anonymously reporting data to improve the above.

If those three things happen I could see a ddwrt type of pack being developed.

There is little incentive to hack GPS or PDA devices:

• Map data is expensive. Free data is mostly unusable for routing.
• Quality navigation routing algorithms are expensive and/or unavailable.
• Open source routing isnâ(TM)t as usable as paid options.

One that is done and a certain mass of users are using the above data and algorithms; maps would become dynamic information sources constantly updated with road changes and maybe even real time road closures, road speed data, one way street information and other valuable tidbits. More advanced devices with accelerometers could make that data even clearer for eventual technology like eco routing.
If you have to have a hackable linux navigation solution now get a cheap umpc or netbook running igo 8 or another nav engine on wine.

According to a direct call to our suppier at VIA this is an engineering sample only. This unit will not be in production until July. We (all hobbyists at http://mp3car.com/) are all very excited to throw this board in our cars. I am glad someone was able to get their hands on one for a test.