I read somewhere that the SD slot is for adding maps (i.e. europe, asia, etc.); w/ regular TT3 you'd have to dump what's on the drive and replace the content on the drive with the other maps you want.
You can also do what I did... Install a 2GB card ($10-$15) and install the 7.10 maps and virtually every POI list, map coloration file and voice you can find without having to worry about running out of space like you would on the built-in memory.