It would be nice to get Terraserver tiles onto your Garmin, but unfortunately it's not possible.
The Garmin units can only display vector maps, while terraserver maps are raster (i.e. images).
So unfortunately this is one feature request we cannot fulfill! Sorry!
There is a new unit out by Delorme that can display raster maps (albeit slowly). I think the unit is getting somewhat mixed reviews, but it's a step in the right direction. In a few more years we'll be able to look at TF style maps on a handheld GPS. Hopefully.