iAds officially start to go live today, July 1st, and Markus Nigrin has started up a list of apps running Apple's official ads already. If you want to see Apple's new solution in action, you can head on over and see if you have one of those on the list. Removem's free version is one of those, which makes sense, as developer Matt Martel told us at WWDC that he was very interested in his options with iAds and other ad platforms on the iPhone. David Barnard of App Cubby also recently released a free mirror app for iPhone 4, and iAds should be in there as well.
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.