For hacking, nearly any Series 2 (i.e. with USB ports) DirecTivo will do except for the R10, which is pretty much the only DVR being provided by DirecTV and their installers right now. You may find some sat distributors/installers with non R10 boxes, and Ebay has a number of units.
Thank you so much for the excellent info. I have learned alot from you.
A quick follow-up question if you don't mind.....I knlw that my RCA DirecTivo unit is Series 2.....How do I tell if it is R10 or R15? (it is a little less than a year old).
Also, there are 2 USB ports on the back.....So they are enabled by the hack I assume?