This is a pretty remarkable service, but there is one awkward caveat mentioned in the article:

"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]