My situation is somewhat different, but it may help.
I wanted to install an upgrade version of Win7 from a retired PC onto a BootCamp partition (I'm running Snow Leopard), but didn't have an existing OS to upgrade.
I created the partition with Disk Utility, not with Bootcamp, and specified the DOS (FAT) format. I'm not sure if that's relevant, but that was my procedure, in case it is.
I tried installing from a (very old) XP (SP1) disk, which isn't supported. It installed, but I couldn't get IE to connect so I could upgrade to XPSP3. I finally hunted thru the drivers in the Boot Camp folder on the OS X disk and manually installed the ethernet driver. That allowed me to connect to the Web.
Then I downloaded the XPSP3 update, but it wouldn't install. Remember - it's not supported, so I was on my own.
I did manage to get MS to accept the authorization code though, so I had a valid (although obsolete) XP installation.
I tried upgrading from XPSP1 directly to Win7, but that didn't work. It said I couldn't upgrade from XP SP1 to Win7. I figured I'd just do a clean install (without first wiping XP) and use Win7 for 30 days until the upgrade was no longer valid. Lo and behold, after installing I entered my Win 7 activation code when prompted and it worked! The authorized (way obsolete) XP install was recognized by the Win7 upgrade and satisfied the 'upgrade' requirement. I now have Win7/OS X 10.6 dual boot without spending a dime!
Good luck. I hope some of this helps you.