Download the latest 10.8.3 Combo Update and install that. that may include the newest version of boot camp where as the Software update version may not.
If that doesn't do it then run Boot Camp Assistant with a Win 7 DVD in the drive and when it gets to the partitioning phase eject the Win 7 DVD and insert the Win 8 DVD.
Als download the version 5 boot camp driver package and save it to a USB thumb drive.
Thanks... That's what I did (well not actually... I just ignored the warning...)... I posted this in response to someone else's thread but thought I would mention here:
I was able to get it working as far as I can tell. I just ignored the only Win 7 supported message and continued. Then I downloaded the latest BootCamp ZIP file from Apple and put it on a USB drive. I got through the Win 8 Pro install just fine. I then ran the BootCamp setup.exe from the USB drive and it did its thing.
That seemed to work OK except my machine was HOT. I mean REALLY hot. I found some others that said to install the actual Windows drivers for the Nvidia 650M so I did that. Machine cooled down a lot but the screen was SO dim. I couldn't use it well. It would be fine on the login screen and then after loging in DIM. I could use the Apple brightness keys (1, 2) just fine but it was set at 100% brightness (even though so dim) so no good. I poked around in Win 8 a bit and turned off the setting that has Windows choose your brightness for you. Now it works great!
Machine still gets warmer than when in OSX but livable I think.
Next step was to try and use Parallels to let me access the BootCamp partition from inside OSX. No go. During the process in Parallels 8 you get a warning that the OS isn't supported and it may screw up your licensing. Oh well. One step at a time.