iPhone users who are looking information about jailbreak often heard about there is a slightly different jailbreak for iPhone 3GS. This is because Apple has made changes on the BootROM to strengthen the security; this significantly adds difficulty to the jailbreaking community. Although the new boot ROM doesn’t mean it is not breakable, just will take slightly a longer time to jailbreak.
The OLD bootrom refers to the boot ROM that produce before last September, version number is 359.3, and the NEW bootrom is 359.3.2 or later.
Here is the method to check your Bootrom version: