Angry Birds is available on more than one platform, and if you own it on more than one platform – say because you gave up your iPhone for an Android handset or graduated to the HD iPad version – you have to complete levels you've already completed. Hundreds of them.
Earlier this year Rovio promised to provide a syncing service, allowing Angry Birds players to save progress across several platforms. Thanks to a tweet spotted by The APPera we have a very approximate release date. Responding to a question about Angry Birds Sync on Twitter Rovio revealed, “we are working hard on that, before summer is the plan.”
Unfortunately we don't have any more details. Angry Birds Sync may only allow you to save progress across the two iOS versions of the game, or it may allow you to sync across other platforms as well. We've asked Rovio for clarification.