f0recast is a very hand tool to check whether your iPhone/iPod is unlockable and jailbreakable. It can also be used to know the baseband and bootloader version of the phone. Few hours ago, iPhone Dev Team updated the PwnageTool for hacktivation or carrier unlock on iOS 4, but it supports iPhone 3GS with OLD BOOTROM.
So if you want to use the Pwnage Tool, f0recast can help you a lot. Earlier the Team released Ultrasn0w 0.93 to unlock iOS 4 for all basebands.
forecast is available for both windows and mac users. You will need iTunes 9.2 for windows and iTunes 9.0 or higher version for Mac. Install this little app, and connect your iPhone/iPod with the PC via USB cable and get the information.
You can download f0recast here.