It happened in less than 24 hours. A few minutes ago, MuscleNerd from the Dev Team posted this tweet that proves that the jailbreak for iPhone OS 4.0 is already possible.
There's a video after the break, with Cydia and Veency running on a 4.0 device. Now we only have to wait and see what the jailbreak community will come up with, considering all the features that Apple apparently "borrowed" from Cydia, like multitasking and inline spell check. We'll see.
Federico is the founder and editor-in-chief of MacStories, where he writes about Apple with a focus on apps, developers, and mobile software. He can also be found on his two podcasts – Connected and Virtual.