thats the case.. there is a jailbreak unlock coming up for 3gs new bootrom owners if they have 2.1.2 bloobs saved.. my one came with 3.1.3 so i want be able to update to it yet,,, :(
http://www.iphonehacks.com/2010/07/how-to-jailbreak-ios-4-for-iphone-3gs-new-bootrom-with-shsh-blobs.html