It works much easier than that.
Once it is released. You enter your Serial into the Mac Up To Date Page:
http://www.apple.com/osx/uptodate/
They validate your Serial Number was purchased during this window, and they email you a code you can redeem in the App Store. When you redeem the code it is associated to your iTunes account and can be downloaded on any Mac it is compatible with.
The code is generated instantly. I did this with my current MBP going to Lion. Too about 2 minutes to do.