As far as I know, it's not available on the servers yet, but if you're excited like me to upgrade to
iPhone OS 4 iOS 4, you probably want to know What you must know for iPhone upgrade day.
Biggest things that are pissing me off already are that 3G users are left out in the cold for multi-tasking, which I can understand if the processor and memory are more limited, and the wallpapers, which I can't fathom. I'm as much of an apple fanboy as the next guy, but stuff like that is just stupid.... I have no idea how putting a wallpaper on the 3G wouldn't be possible, other than Apple wanting to deny users something that is cool and more importantly, visible, to give them another reason to upgrade to a new phone. They did this with 3.0 with another very minor feature, not allowing 3G users to show their battery remaining as a percentage instead of a bar, which is another thing where there's no technical reason for it not to be available on older hardware.
Update: OK, so here's an explanation of why the wallpaper is disabled on older iPhones.