well...
There are a couple ways this could be done.
1. It takes the current backups of the apps that are going to icloud and syncs them between your icloud devices. I haven't seen if this "just works" yet.
2. You can write the save game files to the icloud documents API and sync it around icloud like pages does with your pages documents.
number 2 would require the devs to write to these API's. I hope that it's solution 1, because then it's baked in and devs don't have to change anything.
I never realized this was an issue until I got my first iPad. And yes it's the angry birds issue. On my iPad I barely have any AB completed, while on the iphone i have 3 stars. SYNC!