The move is part of Apple's "App of the Week" promotion, meaning it won't be permanent. Neither program has been free before, although they have both seen price changes. The last price change for the iPhone version was Feb. 12, 2010 when it was reduced from $1.99 to $0.99. It has stayed there since.
For the HD version, the price has jumped around, both up and down. The last price change took place just a few weeks ago, on Feb. 11, 2013, when the price rose from $0.99 to $2.99.
This is the first time the full versions have been free. Since January of 2011, Rovio has offered free sample versions of the game in the App Store, but never the full version. Of course, the Android version of the app has long been free, but that version is ad-supported while the free iOS version does not currently contain in-app ads. There are also paid, non-ad-supported Android versions.
The move came as -- for the first time since May 2010 -- it was noticed just a few days ago that there are no Rovio apps in the top 80 highest grossing iPhone apps. While it's obvious that Rovio is not a one-hit wonder, it's still somewhat of a shock.