Tried adding ShareKit to an Xcode4 project with a target of iOS 4.3 and get 'Cannot find interface declaration for UIWebView' in UIWebView+SHK.h. If I comment out the contents of those files (I'm not going to be sharing from a UIWebView) I get 80 or so compile errors.
I wasn't able to drag files from the sample project into my project for some reason (an Xcode 4 thing, maybe?), so I added the files directly from the ShareKit folder in the finder. Could that be the root of things?