Tiptitude is also supposed to be running iAds, but according to the two reviews on iTunes right now, a) people are downloading the app just to see iAds in action (which is pretty wild), and b) they might not be working anyway. I would say that we'll probably see widespread adoption of them before long, so you probably don't need to hurry up and download an app just to see the free advertising embedded in it. But if you have some free time to spend checking out ads, there you go.
Our own Michael Jones is reporting that he's seeing iAds continue to play sound even if the phone is muted; if you're seeing the same bug/feature, please advise.
Update: Erica Sadun reminds us that if you're trying to refresh apps to see iAds, you'll need to restart the app completely. Press and hold on the app's icon in the multitasking screen, tap the minus to kill it, and then relaunch it to see if the ads are different.