Hi, iAd seems to require iphone os 4.0, so I install the new beta ver xcode and sdk. It works.
But The previous sdk disappear. Now, the new build app (with iAd) is also retro compatible with 3.1.x? Or with iAd I have to renounce to retro compatibility?
When xCode 3.2.3 is officially released you will be able to build to previous versions again, however iAd will probably not compile on pre 4.0 builds.
iAd doesn't work with previous version (before 4.0) I found a solution in adwhirl.
So I put inside my app adwhirl. I put admob, adsense or other circuit and iad. I weak link in xcode project the iad framework, and adwhirl understand if iad is supported, so if ok iad appear else other circuit work!
You should use the standard method of building a single binary which can take advantage of features available in newer OS versions only, but still run on older OS versions too: build the application with Base SDK set to 4.0, and the Deployment Target to a lower OS version, for example 3.0. The frameworks available only in the newer OS versions should be weakly linked and the application should then check at runtime which features are available (iAd for example) and act accordingly.
See Apple's on example on how to do this: http://developer.apple.com/iphone/library/samplecode/MailComposer/Introduction/Intro.html.