"OnLive has no other games in the pipeline that are Windows-only, and we do not expect to have any others."
As they point out, this is the only game that's not available cross platform. It's understandable that this would happen during the beta period, while they are still trying to prove the model to game developers.
I wonder if the restriction is because EA already licensed the Mac publishing rights to someone like Aspyr?
"Unfortunately, because of licensing restrictions, we can only offer Mass Effect 2 for play under Windows. So, if you do not have access to a PC, your only option to play it on a Mac is under Windows using Boot Camp or a similar system. We apologize for the inconvenience. OnLive has no other games in the pipeline that are Windows-only, and we do not expect to have any others."
D'oh!
[edit: As a Canadian surfer, I run into US services and sites every week that don't allow non-US users. Netflix, Comedy Central, etc. I could imagine the licensing issues could be similarly irritating with OnLive as well]