I haven't checked in a while, but last time I did, you could hack the Series 1 boxes, but not the Series 2. But I've heard rumors of mod services for Series 2.
I feel that for the most part, the boxes are commodity items, the value is in the software, which is mostly the same. The Tivo branded boxes come with the real Tivo remote, which is the most ergonomic remote ever, to many.
At any rate, I'd check here first:
http://www.tivocommunity.com/tivo-vb/
Lisa