I have a little project page to document the creation of a 120GB upgraded Apple TV. Take a look, it was fun and challenging to do the first few.
http://www.powermax.com/appletv-take-apart.php
http://www.powermax.com/appletv-take-apart.php
someone feel free to correct me if I am wrong but if you were to buy this unit with the upgraded HDD the HDD would not be under warranty and its possible the whole units warranty would be voided through apple.