There's another aspect of this not mentioned. There is a non-zero number of iPhones jail broken for the purpose of running cracked apps. There is probably a non zero number cracked just for that purpose and that purpose alone.
I would hope that as open developers we can acknowledge that this isn't respecting other people's copyright and is one activity that should be shunned.
Beyond running cracked apps, I don't really see much point of jail breaking an iPhone. The list of things that people can't do that they might want to do isn't that long and over time it's been getting shorter. People forget you couldn't even develop 3rd party apps for the blessed thing a year ago. The apps that were once getting rejected is getting far shorter than it once was. Besides given the apple SDK you can certainly create, run and install whatever.
I think there's larger problems here that are more deserving of time and attention. Is a jailbroken iPhone the solution to the way carriers sign you into a service plan in the US? Nope. Or break the carrier + hardware monopolies? Nope.
All that said, I'd like to see the DMCA struck down as much as the next person. Seems like politically these are the times to give it a try.
If Apple hauls someone into court using the DMCA for a jail broken phone, I'm going to pay careful attention for the reason of the complaint. It's easy enough to identify a jail broken phone after all. If it's just on account of a jail broken phone, ok yeah that's an evil use of the DMCA. If it's because they were say ripping off the App Store, then yeah, that's a legitimate complaint.