Which is about 10% of all iDevices, so not particularly useless. (And, as others gave said, that doesn't even include the developers who can sign and install it for themselves, as well.)
80% of the whole iPhone user mass is a gross overestimation, but I don't think that it's an overestimation to say that 80% of huge geeks have jailbroken their iPhones.
For those interested in Bitcoin right now (mostly geeks) I'd say the percentage of jailbroken iDevices is much higher, i'd throw 80% out as a guess.
I think we can all agree that this is reasonable.