Have you checked the software available on the palm site:
http://us.store.gc.palm.com/?i=1&t1=GPS+Units&u1=t1
Interesting timing for this question. Navteq (the company that provides the map data for most of the navigation systems used today) just announced earnings the other day and I listened in on their earnings conference call.
Their earning were through the roof, (and so is their stock, ticker symbol: NVT) they indicated that the: automotive market, Portable Navigation Devices, cell phone services (like Verizon's VZN), server based markets were all up, the only market for their maps which was down (almost completely dried up) was the PDA market.
I used Mapopolis on a tungsten C a few years ago and loved it. Mapopolis has dropped out of the business.
http://www.mapopolis.com/nlogin.jsp