Cannabalt for iOS now open source

I await a fork that lets me, you know, win.

No; in fact, the game images and sounds are not free, but everything is open source and available for looking at (except the high scores code, which they left out to prevent cheating (explained in the blog post)).

Big difference between open-source and free, at least as you mean it. As the Readme says:

"The code contained within the 'flixel-ios' folder is licensed under the terms of the MIT license."

which allows redistribution. The next paragraph says:

"All other source code remains copyrighted and licensed by Semi Secret Software, LLC under the original terms. See the Game license (filename "GAME_LICENSE.TXT" in this archive) for more details. With the exception of code contained in the 'flixel-ios' folder, you cannot redistribute our source code from the original game. You can use our source code for personal entertainment or education purposes."

So, the code in flixel-ios is FOSS (you can redistribute it,) but the code outside of that directory is source available only (no redistribution.)

Edit: Proprietary software is "...computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, but restricted from other uses, such as modification, further (re)distribution, or reverse engineering.

"Closed-source software, or software distributed without its source code, is proprietary."

It appears to be an iOS port of the Flixel game library for Flash: http://flixel.org/

Interesting, I'd comtemplated writing a similar game using http://anscamobile.com which is lua, but looks and works a lot like actionscript.

Now there will probably be a dozen other similar games though. ;-)