By going to this site in your mobile browser you can opt out of interest-based ads, i.e. ads which gather analytics about your interests and what iAds you've watched. This does not opt out of the ads, but restricts some of the information that is gathered from them.
You need an iOS 4 device to do this (they're the only ones that get iAds anyway) and you'll need to repeat the process on each device.
[crarko adds: I haven't tested this one, although I did try it with my Mac and got an error that it wasn't running iOS 4. It's not clear if visiting the site a second time with the same device would reverse the process and opt you back in. I believe iAds are scheduled to launch July 1st.]

