Geocaching is fun. Mlord can attest to that.
Here in the US, I got for the harder finds - Benchmarks...
For commercial side of it, I'm looking at Rand Mcnally's "Topo Sync!" software.
Supposedly it does just that, compatable with their "Topo!" series CD-ROMS which they sell of most of the US.
The shareware ones out there now it looks like you have to.. 'calibrate' - from what I gather, you supply a BMP/GIF/Image and somehow have to calibrate it when you load it on, so the Palm software knows it's "range" and "zoom" so it will correctly display the map when you move from your starting position. I don't know what 'calibration' consists of yet, I don't want to have to do any (much?) fancy calibratin' whenever I go to a new section of the state and have to load in a new map..
Me.