Well, I can tell you that it works like a charm! Who needs the iPad 2 now when you can get HD quality screen mirroring from your iPhone 4 using this little Apple adapter! I am still on 4.2.1 and loving this! You just need DisplayOut which I purchased on the Cydia store.
So, to clarify: without jailbreaking and installing DisplayOut you can't mirror the homescreen, right?