the "Other" category seems go grow significantly over time, and is not necessarily files stored by other apps, some say it's corrupted data. But it can be eliminated by doing a restore.
for example, my iPad and iPhone both had several GBytes of "Other" data on them. Both are non-jailbroken, BTW. with the release of iOS 4.2 I took the opportunity to do a restore from backup after I did the original update.
the "Other" category on both iPhone and iPad is now down to a couple hundred Kbytes, whereas all my other regular data is still there.
I'm convinced there a bug in iTunes or iOS that didn't let this cruft get cleared out during normal sync. I'm curious whether this behavior changes now with iTunes 10 and iOS 4...