Here's one for early adopters: after I upgraded my iPhone to iOS 5 and resynced all my data, I noticed that the “Other” category on the drive had ballooned from less than 1 GB to over 10GB, effectively filling up all the empty space on my phone. I saw it after iTunes told me it was syncing album artwork and something called “Genius data” to my phone.
Users on that thread report that you can fix it by un-syncing all your music or movies (clear the Sync whatever checkboxes for the device in iTunes), and then re-syncing them one at a time.
That bites since syncing 20+ GB of music takes several hours, but I can confirm that it worked for me. I would plug in your device instead of doing it over wi-fi to speed things up.
(Also, having to totally re-sync after an OS upgrade? That's crap. At least, in theory, it will never happen again since updates are now incremental and over-the-air.)
Anyway, hope this helps anyone else that runs in to this.
Comments? I don’t do open comments. Life is too short.