It would be interesting to note trends in jailbreaking activity against iOS versions and any potential changes to functionality for App Store apps.
(1) Users ask "why am I getting this sandbox error message?" and we say "That happens sometimes on jailbroken phones. We can't fix it."
(2) We have to step up our game re: preventing cheaters, keeping in mind that jailbroken users can change their UDID, fake IAP transactions (but not the receipts!) and modify files on the device.
In a single-player game we wouldn't care about (2) so much, but in a multiplayer game we don't want them breaking someone else's game or polluting our statistics. I don't have numbers, sorry; we don't count the number of jailbreakers, just the number of cheaters.