Well for starters, their server clusters would be secure. Just as hackers have a hard time getting into all the other server clusters that businesses use (Google, Microsoft, Apple, etc), they would have a hard time getting into these, and would require a very good hacker at that.
I would get OnLive if it comes out and actually works. I have plenty of doubts though, so I guess it is a wait and see type of thing for me. Latency (although I have super fast internet) seems like the real issue. Plus, if they want to host multiplayer games, the farther the distance, the more the latency so with all the controller information and video streaming back and forth, I would expect some pretty drastic delay.