Are there (good) ways to deliver paid-for ads to iPads running iOS3.2?
iAd works on iOS4.0 and higher, and AdMob seems to support only the iPhone/iPod Touch devices, not the iPad.
AdMob supports the iPad, and they provide a couple of different banner sizes specifically for it: 320x270, 488x80, and 748x110, all defined in AdMobView.h. If you don't have those options available, you probably need to download an updated version of the AdMob SDK.
Every iPad running iO S3.2 is eligible for a free upgrade to iOS 4; the situation is different from iPhone and iPod touch, where various versions co-exist.
So, I don't think you have to care for iOS 3.2 users on iPad. I think people who buy apps on iPad would surely have updated to iOS 4 by now.