Never noticed this before, thanks Lunatik for posting the link.  Go there with your iOS4 device to disable iAds!

To opt-out of iAds on a device running iOS 4, you must enter the following address into the device's web browser:

http://oo.apple.com

If you have multiple Apple mobile devices running iOS 4, you will need to opt out on each device.

If you still encounter difficulties, please visit the support website at: http://support.apple.com/kb/HT4228