I play Angry Birds on my iPad and on my Android phone. Because there is no way to synchronize my progress between the two devices, I only play the original version of Angry Birds on my iPad, and the Seasons and Rio versions on my Android smartphone. Rovio has revealed in a tweet that they are working on the ability to synchronize across all platforms, which will enable me to not worry about which device I play the game on.
It sounds to me that Rovio is working on providing a feature that is built-in to X-Box Live on Windows Phone, which has the ability to start playing a game on the phone, then pick up playing where you left off on a PC. Because many people now own tablets and smartphones, not to mention personal computers, the ability to synchronize game progress across all platforms will be very useful.
Rovio did not provide a time frame for when we will see the synchronization capability, but indicated that it may be coming soon. My guess is that the synchronization functionality is part of a back-end infrastructure that Rovio will use to expand their game franchise.