Once a phone is exploited to accept custom firmwares, you can always update with one even if there are no JB for that firmware. For example, lets say 4.2 comes out and all current JB exploits don't work, if you can create a custom firmware for 4.2, you can still update to 4.2 given the phone was previously pwned JB. That's why these people get to update to newer firmware within hours and days of Apple's official release. While, others have to wait for new JB.Also I keep seeing stuff saying pwnage tool only works if the phone has been previously jailbroken??? What is the deal with that?
Since Pwanagetool was updated to work with 4.1, you don't have to be previous JB to use it.