Apparently Apple has a new trick up its sleeve that may block jailbreakers. Whenever you restore your iPhone, which typically happens when you jailbreak an iPhone, iTunes now obtains unique signatures for your device from Apple's servers.
So it may be necessary for you to find these numbers as a future jailbreak may depend on them. iClarified has done the dirty work and come up with some guides to help you find them. Mac version here. Windows version here.
It is not clear whether you, the end jailbreaker, will ever truly need this information, but the jailbreak Gods advise it as a good precaution.
And remember, if you do plan on jailbreaking at some point, wait for the go ahead from those same jailbreaking Gods before updating to 3.1 when it arrives. We'll let you know.
