First, a quick reminder. There are two different tools that the iPhone Dev Team is offering. One jailbreaks the iPhone, and the other unlocks it. Jailbreaking lets users install applications outside of the iPhone App Store via Cydia. Unlocking lets users remove the AT&T SIM card, and use the SIM card from another mobile network operator, such as T-Mobile.
For those wishing to jailbreak the iPhone 3G or 3GS, there is good news and bad news. The good news is that PwnageTool 4.0.1 works with previously jailbroken iPhone 3Gs and 3GS's that are still running the old bootrom. The bad news is, it doesn't work with un-jailbroken iPhones. In other words, if you're a serial jailbreaker and have been careful to avoid the iOS updates thus far, you're in good shape. If you restored your iPhone 3G or 3GS to "factory" conditions with iOS4 on Monday, you're out of luck. The iPhone Dev Team offers full instructions here.
As for unlocking, the news is a bit better. According to the iPhone Dev Team, right now ultrasnow can unlock all iPhone basebands from 04.26.08 and up. The baseband is the wireless radio that connects to AT&T's network. The baseband software has been updated by Apple over the years, each time making it more difficult to unlock the iPhone. The bad news is, you have to jailbreak your iPhone to use ultrasnow to unlock your iPhone.
Sounds like fun and games, right?
At this point, I can understand why some might feel the need or desire to unlock their phones from AT&T's network, but jailbreaking has less and less appeal as the iPhone gains more features. The biggest reason hackers broke into the original iPhone was so that they could install additional applications. Now that there are over 200,000 applications available in the iPhone App Store, there doesn't seem to be as much need to take this drastic step.
However, if you like to tinker, and don't mind take chances with your expensive iPhone, by all means give it a shot.