In a bid, to fightback the hackers exploiting the 24kpwn exploit that allowed the iPod touch 2G and then the iPhone 3GS to be much more easily Jailbreaked / Unlocked, Apple is now shipping the newer models of 3GS with an updated boot rom that is resistant to this exploit. What this means is that the value of older iPhone’s circulating in the market is much more from a jailbreaking standpoint.
Neither Geohot’s Blackra1n nor Dev-Teams Pwnage tool currently work on this updated boot rom i.e. iBoot-359.3.2. To check which bootrom your device is running you need to put it in DFU mode and connect it to the mac. You would be able to see the details under the system profiler there.