Jay Freeman (saurik) has been a busy man lately. for the past two years. for a while now. His latest project to come our way was just released less than an hour ago - a way to finally downgrade your iPhone 3G[S] from 3.1 (with a few "well, if"s in there) to something currently jailbreakable (like 3.0.1).
saurik's got some crap to toss on Apple's wall, too, with this one. The guide article features not only an informative look into the methods of jailbreaking, and a history of Apple's push for individuality (and of course the instructions on how to downgrade your iPhone 3G[S}) - it goes so far as to offer up some tasty tidbits like "Congratualations, you just overthrew your orwellian overlord, and have taken back control of your device," and "Apple, as a company, has turned into a corporate hypocracy, embodying the very ideals that it claims to be rebelling against."
It's a great read with some solid points - if you're at all interested in this stuff, you should read the whole thing.
The gist of the situation is this - people can't downgrade their 3G[S]s from 3.1 (until now with this new method) because Apple has implemented a new security method. When you restore an iPhone 3G[S] in iTunes from 3.1, iTunes will show
"Verifying restore with Apple...", during which time a challenge/response protocol is used between the iPhone and Apple: a "partial digest" of the firmware files being used is sent to a server, which can then decide to sign off on the result... or not.
A big, gaping stopping point, at the moment, is if you didn't press the "yes i like jailbreaking" or wtvr that link said in Cydia for the few days it was there (over 50,000 3G[S] users did), you don't have an ECID SHSH "on file" with Cydia's servers - meaning you can't downgrade. This will only work if you pressed that link in Cydia while it was there (before 3.1 came out). Read saurik's article for full reasoning behind it.
An exploit is [supposedly] available for 3.1 though, so you will be able to jailbreak 3.1 anyway soon. However, this doesn't mean you don't need your ECID SHSH "on file" with Cydia - you still should (when the jailbreak comes up for 3.1 you'll be able to get it on file again). Because as of 3.1, you can NEVER downgrade without this method (yet) unless Apple specifically signs the firmware. Meaning - they decide what firmware you can use and can't use.
The instructions in saurik's guide, while straight-forward, are a bit more complicated than "plug in, press the Easy button" - so be warned, it's a fun throwback to the earlier days of iPhone modding, when you had to actually DO something.
Read the full guide and article here, and ask questions in the iPhone 3G[S] Downgrade forum.