I apologize for the clutter but I want a new message to include in the FAQ to answer this question directly.
Back in November, Iron gave us a list of the files that Mobile PC can support. I apologize that I didn't have enough experience with Mobile PC at the time to fully understand what he was suggesting. He was correct and I think it's useful enough to get this into its own thread and into the FAQ.
Garmin Mobile PC will use all of the following four files if it finds them in the GarminMobilePC folder.
1. GMAPBMAP.IMG - This is the basemap.
2. GMAPPROM.IMG - This is the preloaded maps that come with Mobile PC.
3. GMAPSUPP.IMG - This is a compiled map file, usually created by MapSource. It can contain segments from as many different map products as you like (with some limitations). See the thread about using MapSource to add detail maps to Mobile PC.
4. GMAPSUP2.IMG - This is another compiled map file, the same as #3.
Mobile PC will use all, or any combination, of these files if it finds them in its home directory.
Thanks to Iron for a very useful addition to our knowledge of Mobile PC.
EDIT (08SEP2010): Mobile PC v5.00.70 no longer supports GMAPSUP2.IMG
..... (02NOV2010) As cooperman reminds us below, 5.00.70 now supports GMAPPROM1.IMG so if you need a fourth map file you can just rename GMAPSUP2 to GMAPPROM1.
...ken...