I'd love to say that the quality of Apps on the Apple App store is so superior to those made for jailbroken iPhones that no one would bother jailbreaking anymore. However, this is definitely not the case. While a few of the apps are quite polished a lot are on par with their jailbreak equivalents and many jailbreak apps will never make it to the Apps store as they will upset network providers or apple - e.g. the app that allows you to create multiple iTunes library's on the phone so you can sync with more than one computer, or VoIP apps that allow you to make calls without getting charged a penny by your mobile provider.
Add to this the fact that most of the apps on via AppTapp on a jailbroken phone are free and it makes jailbreaking your new iPhone 3G very tempting. Some have already managed it as seen in this video:
So what are we waiting for - lets unlock our phones and get lots of free App goodness. BUT WAIT Jailbreaking was tollerated before and some of the jailbreak app developers have been welcomed on to the App store... however, before Apple had no competing service to rival the jailbreak apps and now they do. They will not want users to be able to choose between a free jailbreak app when they are trying to sell a similar App for $9.99 in the App Store.
So what will Apple do. Expect a lot tighter policing of iPhones. Regular updates closing loopholes that hackers have used to jailbreak, and even bricking of phones that it detects have been tampered with. While not illegal it will be a lot riskier jailbreaking your phone this time round.