Could an upgrade edition of Windows be used to install boot camp if you connect two external DVD drives? I found the following FAQ on Apple's website:

Why can't an upgraded version of Microsoft Windows XP or a full version of Microsoft Windows XP that does not include Service Pack 2 (SP2) or later be used for installation?
An upgraded version of Microsoft Windows XP cannot be used because you are required to validate your copy of Windows by inserting an original Windows CD, but there is no way to eject the first disc until after Windows installation is complete and the Boot Camp drivers from the Mac OS X 10.6 DVD are installed.

And so I was wondering if you had two DVD drives hooked up to a MacBook Air, could you install boot camp using an upgrade version of Windows 7?
What are you trying to upgrade it from? If the answer is WinXP, then no.
From Apple's 10.6 bootcamp install pdf:

Upgrading to Windows Vista or Windows 7
You can upgrade Windows XP to Windows Vista or Windows Vista to Windows 7.
Note: If you’re using Windows XP, you cannot upgrade to Windows 7. You can perform
a clean install (also called a custom install), which erases your Boot Camp partition.
Before you perform a clean install, make sure you have the installation discs for all your
programs and a backup of your personal files and settings. You can restore them after
the installation.