Apparently if you are purchasing a new iPhone 3GS it will come with a new bootrom which blocks the 24kpwn exploit according to a report by iClarified states the iphone blog.
Last week shipping began of iPhone’s with iBoot-359.3.2 bootrom and are currently being sold in AT&T and Apple stores, which means if you get one of these handsets you won’t be able to jailbreak it until a jailbreak solution is found.
Anyone wanting to jailbreak an iPhone you’ll need to locate a refurbished iPhone 3GS or locate older stock, but no doubt someone will break this block before too long.