The upgrade DVD I bought allowed me to do a full clean install on my Macbook Air. The disc doesn't even say upgrade or anything to differentiate it from the one in the Mac Box Set, beyond a slightly different part number. So I think Apple is just expecting their users to be honest and buy the proper version this time.

I have an up-to-date disc on the way for the Air, not sure if that one will be an upgrade only, or an identical copy to what I bought for my other machines.

They did change the install process though, eliminating the manual prompt about what type of install to do. You will need to boot to the DVD, then run Disk Utility from the Utilities menu to wipe the drive.

Also, looking forward to making use of this feature: Wake on Demand. Pretty cool how it works, the Airport Base station runs a Bonjour proxy. If it sees a request for a bonjour service on the network and the machine is asleep, it takes care of sending the wake on lan packet out. This includes wide area bonjour requests, aka Back to my Mac. This works for Apple TVs as well.