It can be done pretty easily with a jtagged 360, as long as it's a 2.5inches drive. 3.5 drives also work but you gotta mess around with the 360. Then you can dump every game you want on the hd and play with them without having the goddamn dvd in the drive, hell you don't even need a dvd drive.

As for the new 250gb, yeah, they pretty much changed the hd casing, that's about it. People have put their old 360 hd in the new 360-s, with the help of some tape to fix the casing after you open it.