Transfer Save Data between two iDevices? Back it up to Mac?

Let's say I have a game save, and want to continue playing the game on someone else's iPhone. Is that doable? Can I back it up on my computer and then reinstall it later, if I delete the game from my phone for a time?

You'd have to create a save game system using the web, where you'd upload the save from one iPhone to your server and then download it on another, presumably using some kind of player profile.