Ad Hoc distribution for beta testing xcode 4.3

I've had this exact problem even where, like you (as far as I can tell), everything is done right. What I discovered is that if you've EVER had the url to the plist file or the app file incorrect then your iPhone will always fail to install. There's some kind of cacheing going on where the phone fails to download the updated plist file if it's previously downloaded an incorrect one.
The fix I've used is to try a different phone (that hasn't cached a previous version), or to make a new version at a different URL. Sounds crazy, but it worked for me!

The only difference I can see is in the CodeSigning, ensure both lines under Adhoc point to your certificate.

Second thing to verify: You did not mention the plist that is created during the "Save for Enterprice". Did you upload that to your server, too? And does your link point to this plist? Check out this answer for the plist stuff: How to send the build to already provisioned remote person's iPhone