Geniuses can see a debug log (with a beautiful Apple-esque UI) from your phone telling them percentage of calls dropped per day/week/month and what apps have crashed on your phone and how many times they crashed, etc. Warning those of you who want to bring your iPhones in for service that restored clean then restored from backup without any of the Cydia apps, the logs will also be stored in that "Restore from backup," so please beware. Just go in clean with a few apps on it, if the genius asks you why there's nothing on it just say you restored it (it's none of his concern anyway).