Yes, you can hack series 2 direcTivos. It makes a lot of sense to do, because the current sat stream isn't going to go away, they're simply going to supplement it with the new Mpeg4 birds.

The only reason I haven't hacked mine is that I don't want to lose the current content in the process. There's no way to back up current content so that it's watchable post-hack. (As far as I know: Someone correct me if I'm wrong.)