I was just about to post a thread on the same topic!
Wouldn't it have to be released before the iPhone 3G goes on sale corresponding to your country? Because I'm guessing your iPhone will have to be updated to 2.0 in the store, before being functional. I'm gonna say anywhere from 7AM - Noon. I doubt anytime after noon.
Edit: Remember though, you need iTunes 7.7 to upgrade to 2.0 and access Applications, so once 7.7 is available, you'll be able to go right in and download Applications/ upgrade to 2.0