I have a MacBook Pro 13-inch, Early 2011, and have upgraded to OS X 10.8.3. I attempted to install Windows 8 via the "Boot Camp Assistant" utility application, and received the "Your Mac only supports Windows 7" message when attempting to "download the latest Window support software from Apple."
According to this Apple support article, the early 2011 13-inch MacBook Pro should be capable of running Windows 8 (64-bit) with Boot Camp 5: http://support.apple.com/kb/HT5634
Apparently this is a glitch in the Boot Camp Assistant, as I've read of it affecting even newer MacBooks. With the "Your Mac only supports Windows 7" message, I was uncertain that Boot Camp Assistant would download the correct set of drivers for use with Windows 8. Instead I went to this page: http://support.apple.com/kb/DL1638 and downloaded the Boot Camp Support Software there. I followed the directions to copy the contents of the downloaded .zip file to the USB flash drive. Then I used Boot Camp Assistant to install Windows 8 after deselecting the "Download the latest Window support software from Apple" option. After Windows 8 was installed and opened then I opened the Boot Camp folder on the USB drive and double clicked on "setup.exe". Windows 8 has since performed flawlessly on my MacBook Pro, so I believe this to be a suitable work around. Hopefully Apple will soon correct the flaw in the Boot Camp Assistant.