Marco Arment on a subtle but potentially far-reaching change in iOS 5:
There’s no longer anywhere to store files that don’t need to
be backed up (or can’t be, by the new policy) but shouldn’t be
randomly deleted. This is problematic for lots of apps […].
This kicks in when free space is low, so it’s going to affect people with 8 and 16 GB devices more than those with 32 and 64 GB devices.
from Daring Fireball http://www.marco.org/2011/10/13/ios5-caches-cleaning