I needed to install the WP8 SDK so I did just that yesterday, and I have VS 2005, VS 2008, VS 2010 and VS 2012 on my system (amongst many other applications). The upgrade advisor told me there were issues with VS 2005, VS 2008, VS 2010 SP1 and a whole bunch of VS components (like remote debuggers, etc). I did the upgrade anyway, and when I ran VS 2012 it automatically downloaded and installed an update. I tested VS 2010 and VS 2012 and both seem to work fine.
My main issue was that XP Mode no longer works, and I can't enable Hyper-V on my system because it doesn't support SLAT. A warning that XP Mode would stop working would have been nice, but I can live without it I guess.
My system still takes about 3.5 minutes to boot. Before the upgrade it took about 5 minutes. I usually do clean installs but this time I just didn't have the time/energy to go through a complicated re-install of everything (many music applications with dozens of plugins, many with their own convoluted installers). I can live with a slow boot until I'm ready to do an actual clean install at a later time.