I've actually toyed with a game system using two values per tile. One is the elevation, and the other the type of ground cover.

0-9 elevations

r=rocky, f=forest, g=grassy, w=water, etc.

This allows for a slightly more complex computation for movement, as movement is normally slower uphill than down, and a game engine could even use the map to figure trajectory differences for missile weapons to nearby tiles and things like that.

As for using a DB instead of a flat file of keeping it all in memory, I was planning on putting regions encoded like the above into the database, and pulling entire regions from the database into memory when needed, so say you could have a 16-by-16 or 32-by-32 region that gets pulled out when someone is within a certain number of tiles from it, or when someone actually enters it if the game doesn't allow multi-tile vision (depends on style of game and granularity of land areas depicted by the tiles).

I've worked quite a bit in this area, and I'm always happy to talk about it. I may even be interested in joining a project if that's an option. Perl is a good language for this type of game because of the ease of manipulating the data structures in different ways.