Not directly related to the NXT, but potentially very useful for it: LEGO has released a new type of tank-like treads. In the educational NXT kit are a set of rubber fixed-length treads, but a new snowmobile set from LEGO...
has a multi-component tread formed from many individual links. While LEGO has made tread links before, these are much wider, have small cleats on them (should work much better on the carpet), and perhaps hold together better than the old style. More to the point the length of this tread system is very adjustable, leading to variable length treads. Bravo LEGO! I don't have these (yet...), but someone has put up some more detailed pictures up on Brickshelf:
Brickshelf gallery of Technic snowmobile