The new bootrom 3gs is the ONLY iPhone to have a "signiture" within the firmware. For those who don't realize, FIRMWARE is different from OS version. FIRMWARE is the bootrom, OS version is 3.0, 3.1, 3.1.2, 3.1.3, 3.2, and so on.
You see, hardware is the core of any system, on top of that is the firmware, and on top of that is the operating system.
There may NEVER be a jailbreak for the new boot rom 3gs for 3.1.3. It has been mentioned ALL OVER cydia NOT to upgrade to 3.1.3 and it has also been mentioned all over cydia that there is NOTHING at all of use for 3gs to have 3.1.3. It's really only usefull for 2g iPhone owners, as it does have new things for the battery. It was stated everywhere that once upgraded to 3.1.3 you will NEVER be able to downgrade or jailbreak.
Here is the excerpt from wikipedia for credentials:
"Device must be jailbroken on 3.1.2 before upgrading using custom 3.1.3 firmware. Note: Apple no longer signs any firmware besides 3.1.3. Only signed firmware can be installed on an iPhone 3GS, which means once it has been upgraded to 3.1.3, it cannot be downgraded to 3.1.2 and hence, cannot be jailbroken. The only exception is if the device's SHSH file has been previously backed up while it was jailbroken. If an iPhone 3GS user upgrades to 3.1.3 without having their SHSH file backed up, it is impossible to jailbreak the device."
So there you have it, 3gs users with 3.1.3, new bootrom are officially screwed! Go to ebay and buy a used 3gs, old bootrom, and start from square 1 again!
No offence but this thread is quite useless and is a repeat of every noob's question. Sorry, but we are all laughing at you.