Yesterday the Dev Team released the latest incarnation of their PwnageTool, which will allow owners of iDevices with the older bootrom (i.e. not iPod touch 3G or newer iPhone 3GS owners) to create a custom jailbroken OS 3.1.3 firmware file. This firmware file will contain the older base-band so if you have unlocked with blacksn0w you should NOT use it. This tool is for Macs only – there is no Windows version.
Make sure you read and properly digest the full Dev Team post on this release HERE before you even consider using it.