This announcement is not about jailbreaking the new 3G iPhone hardware. A huge attack against the first generation iPhones was discovered in March which completely compromised the secure bootloader and made it possible to install any unsigned firmware image you want.
All they have probably done here is hacked the 2.0 firmware to include the installer app and then loaded it on first generation hardware with the bootloader vulnerability.
I've heard that the belgian iPhones, while "unlocked", only work with Belgian cell phone companies. I don't have any source, but its something to consider and watch out for. Somebody said its called "Nation Locking" or something.
I figured there would be all kind of tricks up AppleT&T's sleeves this time and if you want an unlocked phone, it's probably worth waiting a few months (weeks?) until the proper jailbreaking procedure is figured out.
http://wikee.iphwn.org/news:pwnage
All they have probably done here is hacked the 2.0 firmware to include the installer app and then loaded it on first generation hardware with the bootloader vulnerability.