Update 30-Dec-2008: There have been a lot of comments saying that this doesn't work, but there have equally been comments saying that it does work. Other than saying that "it works for me", which isn't very helpful, here's what I did to modify the BootCamp.msi. Download an MSI editor like InstEd to edit the MSI file. Look in the tables and find references to VersionNT. Version 501 refers to XP. Version 600 refers to Vista. Replace references to VersionNT=600 with VersionNT>=600. This will tell the installer to install Vista drivers and reg keys for Vista and above. For comparison, the version of Windows 7 that I'm running (PDC Build 6801) shows up as 610.
If you're trying to run the build of Windows 7 given out at PDC on your Mac using Boot Camp, you will need to install the Boot Camp drivers just as you would do with Windows XP or Vista. However, the Boot Camp installer checks the OS version string and will fail.
While you can install the drivers individually, you will not get everything. The most obvious missing pieces are keyboard hotkey support and the on screen display. To fix this, I tweaked BootCamp.msi so that it will install Vista drivers for Vista and above (i.e. Win7). This is what you need to do:
Install Windows 7 on your Mac from the installation media. Do this by holding Option down during boot and selecting the optical drive as the boot device.
Once Windows 7 is installed, copy the Boot Camp directory from the Leopard installation DVD to a folder on your Windows 7 install. Note that to eject the DVD, you will need to go to Computer, right click on your optical drive and select Eject.
Download the patched BootCamp.msi from here and copy it over the original that is located in Boot Camp\Drivers\Apple.
Run setup.exe located in Boot Camp and all the drivers should install correctly.
On my MacBook Pro, I noticed that audio was non-functional and the RealTek driver supplied with the Boot Camp drivers did not work. I was able to get a later driver from the RealTek site to work. You can download it here (get the "Vista Driver(32/64 bits) Driver".)