iPhone OS 3.0 Jailbreak: 3G Users Warned
Can the upcoming Apple iPhone OS 3.0 be jailbroken? According to the iPhone Dev Team yes it can, but also issued a warning to some iPhoners who may be just a little eager to grab that copy and paste feature.
The warning was iPhoners who have previously unlocked their Apple iPhone 3G with Yellowsn0w may run into some trouble if they download iPhone 3.0 OS beta.
Now although Yellowsn0w isn’t jailbreaking software it is used for unlocking Apple iPhones and capitalises on the iPhones radio which is used to link the iPhone to the network, and iPhone 3.0 update will change it. So basically if you download iPhone 3.0 with Yelowsn0w installed, the iPhoner will lose Yellowsn0w and not be able to retrieve it.
Source — cnet