I've Windows 8 running in a separate partition. I started with the upgrade assistant in Windows 7 and selected the "Keep nothing" option. After it created the bootable USB stick which I then booted, I chose the "Custom (Advanced)" installation, selected the unpartitioned space on my SSD containing Windows 7 and after the installation completed, it gave a boot menu with the choice of Windows 7 & 8.
As a result, I can continue using my existing Windows 7 or the clean Windows 8 installation. It activated successfully.
While the OS Choice menu looks nice, the only thing I don't like about it is that it reboots the PC to go back into Windows 7. For example, if I want to boot into Windows 7, I must first wait for my PC to POST and load the OS choice screen. When I choose "Windows 7", the PC reboots and must wait for the POST process again before it boots into Windows 7. Finally, when I shutdown (or reboot), the OS choice screen appears again the next time the PC is booted. When Windows 8 is chosen in the OS choice screen, the Start (Metro) screen appears pretty much instantly.