Cool
Lots of maps will be needed, there is lots of Faerun to cover
Perhaps it would be best to "simply" make a super-map of the ENTIRE world - then we can cut it into pieces later
Beware, as a lot of locations are only shown in certain supplements and not in others, so a merging of sorts may be necessary... (I tried this kind of thing for the area around Cormyr before, as there are lots of supplements for that place.)
The Underdark idea is a very good one - they did have an overlay thing before (not digital, of course) - good for figuring out where you are...