No, downloading and installing it on your iPhone 2G won't magically give it a better camera, video recording, or unicorn tear-powered 2x speed. (It will most likely give you heartache -- never mix and match firmware!) but MacRumors forum has already found iPhone OS 3.0 for iPhone hardware 2,1 lingering about Apple.com.
(We're betting it's waiting in line for the hardware -- curse those pre-order limits!)
Again, we don't recommend doing anything with it -- if you're getting an iPhone 3G S it'll come pre-installed. We simply present it as another check box on the road to release, like Find My iPhone going live, and Push Notifications hitting the App Store.
Fun week ahead, no doubt!