Don't accept this answer without vetting it further first, but I think I noticed something during my install.
I think that you don't get any SDKs as part of the XCode 4 install. It was only after I installed XCode 4 and launched it, that the project window showed a download in progress. Clicking it gave me the option of downloading: OSX 10.6 SDK, Legacy OSX SDK, iOS SDK, and something else that I don't remember at the moment. Perhaps extended documentation.
So, the XCode install itself is still very heavy. What with XCode visually, all it's associated applications and libraries, Clang/LLVM/LLDB, profiling tools, and everything.
But, the SDKs themself? I don't think it comes automatically. I can't seem to figure out how to get back to choose the other SDKs to download, so I'm not 100% sure in this.
I don't have a source, but the thing is that the iPad/iPhone AppStore is different than the Mac Apple Store. The iPhone game costs 0.99, whereas the iPad version (optimized for the larger screen) is more expensive, yet these two apps are in the same Store.
Since there's no way (thanks the Apple) to provide free upgrades to the Mac Apple Store for existing customers, Rovio charges you for the game again if you want it in your Mac, no matter how easy it was to port for them.
It was probably not a super hard task, but in terms of administrative work, they have no choice unless they'd wanted to give it away for free, which clearly they aren't.
So the answer is yes, you have to buy it again from the Mac Apple Store.
Best Answer
Yes, you do. They are two entirely separate apps for two radically different devices.