You've got to wait a couple weeks to get this if you're the run-of-the-mill iPhone type -- but if you've ponied up the Benjamin for an iPhone Developer Program membership (who's probably suffered through a handful of iPhone OS 4 beta builds over the last few months), you're now able to download the Gold Master seed of iOS 4 directly from Apple's site. Let us know if you manage to sprout a front-facing cam on your 3GS to test FaceTime or anything cool like that, alright?
Update: We just updated to iOS 4 GM and haven't noticed anything particularly new or crazy yet. The install requires a beta of iTunes 9.2 to be installed as well, which adds in support for app folders and backups, but otherwise life is proceeding just like it did when we had beta 3 installed. We'll let you know if we spot anything as we go.