Apple ads new don't-purge, don't-back-up file store in iOS 5
It introduces a new way for developers to specify files that should remain on device, even in low storage situations.
Previously, developers who wanted to keep data cached on the iPhone or iPad but not have it backed up to iTunes -- because it would create a long delay for their users -- put them into temporary storage. With iCloud and iOS 5, however, Apple became much more aggressive about purging data from that temporary storage, leading to users starting apps only to find them empty and their stuff gone. This was especially vexing when on an airplane or otherwise off the data network and the content couldn't just be re-downloaded.
While Apple doesn't always seem to realize the ramifications of their changes, in this case at least they're addressing it quickly. Hopefully this gives the best of both worlds -- persistent, non-critical storage that's there when you want it but doesn't slow down or eat up storage for your backups.
Source: Apple Developer News