Unfortunately, the OnLive viewer app isn't that app just yet -- it doesn't support actual gameplay. What it does support is OnLive's social networking features, so you can make and contact your friends on the service from the app, and see what they're up to while online. You can also watch "Brag Clips," which are uploaded videos of other people's gameplay, and you can even stream straight from certain live games on the service. That's pretty cool -- it at least shows that the iPad is capable of receiving OnLive's gameplay streams.
But it's too bad we can't yet play games on the iPad. It's possible this is more of a test release, to see what demand is like on Apple's tablet and how it all runs. At least it is free. And eventually, we will hopefully get OnLive's core functionality running on iOS devices. When that finally comes around, it'll definitely be something to see.