Careful - have you actually dug around to see the crap that some apps leave? Crap that appzapper, appcleaner and others do not find? Go hunting - it will surprise you.
Why don't make an application for non-jailbroken that can make backups or copy the savegames to the computer via sincronization. It could give an option which save game to backup. I think apple may accept this way.
That makes sense from a backup perspective, but keep in mind that just saving the data is not enough: you need to have system to restore it in place once you reinstall the app. Realistically you are taking something that, up to this point, has been handled automatically (with the backup and restore system in iTunes) and turning it something that has to be managed manually by the user -- with all the potential for error that that entails. Certainly doable, just not trivial. (In a way, it's a little bit like copy-and-paste.)
There's a section where we choose which applications to install and remove. There, should have an option to create a backup for save games of those applications that are being removed. I think it's possible for them include that option. I don't if they're thinking the iphone as true gaming device.
