Recently I installed Vista x64 on my pc. Everything ran great with the latest versions (as of 12/22/07) of Dem2ToTOPO, IDL VM, GPSMapEdit, cGPSMapper, AND MapSource. However, if you are following the instructions on http://home.cinci.rr.com/creek/garmin.htm, the registry settings listed on the website will not work with Vista x64. I have not tested the 32 bit version of Vista or versions of MapSource previous to 6.13.6.
The issue with Vista x64, MapSource and the original registry edits is that the new map does not show up in MapSource. One is never given the drop-down box to select the newly created custom map nor is the map listed in View->Switch To Product. After doing some digging into the registry, I figured out the entries that need to be made. Below are those entries. Hope this helps.
Vista x64 Registry Edit:
REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Garmin\MapSource\Families\SE_MN]
"ID"=hex:64,00
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Garmin\MapSource\Families\SE_MN\1]
"Loc"="C:\\Garmin\\Custom\\"
"Bmap"="C:\\Garmin\\Custom\\SE_MN.img"
"Tdb"="C:\\Garmin\\Custom\\SE_MN.tdb"