The HD version doesn't appear to be included in the 24 hour free deal this time, just the iPhone (non-HD) version. FDG Entertainment announced on Twitter that they were setting two games to free for 24 hours: Physics Gamebox and Cover Orange. The HD version is still showing up at full price, so I expect that it's not included this time around.
I would gladly pay a couple dollars for this, you will regret it if you don't get it while it's free! Like others have said, it has some similar aspects from Angry Birds. Very fun!