Yes. the iOS 4 has been released! We have already made a post on the iPhone 4 which will start shipping from 24th onwards and have also informed you about the iOS4 and its features. The iOS 4 will be free for iPad, iPhone and iPod touch users unlike last time when iPod touch users had to pay $5 for the iPhone OS 3.0. The new iOS 4 has added 1500 APIs for developers and not to mention multitasking [supported by iPhone (all generations) but only supported by iPod touch 3rd Generation (32GB and 64GB)].
I am pretty sure a Jailbreak is on its way and is just waiting to get downloaded.
If you have a jailbroken iPhone or iPod touch then do not upgrade unless you are ready to comprise on your Jailbreak which will be removed once you have upgraded.
UPDATE: There is no official update about it yet on Apples Website but it’s live. Just open iTunes, plug in your iDevice and click update until iTunes start updating. Just keep hammering the update button relentlessly! Make sure you have iTunes 9.2. If not then download iTunes 9.2 now (mac+PC)