So, jailbreaking your iPhone is now legal. So let's sit back and wait for everyone to jailbreak their iPhones and cut Apple out of the lucrative App Store market ...
Woah there, not so fast!
See,the decision as to whether to jailbreak an iPhone or not rarely comes down to the legality of it. In fact, I'm pretty sure that the idea that it might not be legal to tinker with your own device would cross the mind of the average user. People rip DVDs, download music and video, and grab cracks for games off the interwebz without giving the "legality" of the matter any thought.
The decision to jailbreak or not has little to do with the legality of the move. To jailbreak or not to jailbreak comes down to the technicalities of the process. Is it easy? Is any technical know-how needed? Is it painless? Are there any drawbacks?
While jailbreaking isn't technically challenging, it's not without a certain level of complication. On top of that, firmware updates destroy existing jailbreaks, and so care has to be taken when upgrading. Oh, and there's the ever-present chance of hosing your iPhone the more you mess with it ...
... and if you do that, don't expect Apple to be sympathetic.
What this ruling does is make jailbreaking a legal process, and while that in of itself doesn't make it any easier, it does pave the way for commercial outfits to come in and try to de-geek process. This might help popularize jailbreaking services and take them out of the fringes and into the masses, but even then, I just don't see there being enough on offer in jailbreaking to attract the masses (except, maybe, unfettered access to pirated apps).
Let me give you a car analogy. You can do pretty much anything you want to your car (OK, within reason ... those Boudica swords you've always wanted to attach to the wheels are still illegal), but most people buy a car and run it until they're bored with it. Perhaps they hang fluffy dice off the mirror, or get a personalized keyring, but tinkering with cars, despite being big business, is still only something that appeals to the few.
The masses aren't buying iPhones with the idea of making them better, the masses are buying iPhones because it suits their needs. Jailbreaking is still for the geeks and hobbyists.