I installed Visual Studio 2012 a couple of days ago. I still have Visual Studio 2010 and 2008 installed on my machine as well. When I open my Visual Studio 2010 projects all of my toolbox items have disappeared. I've tried the reset toolbox, rebooting, cleaning the registry entries and letting them rebuild.
I could go back in and "choose Items" on each of my projects, but there has to be a better way, and a reason this happens. I've seen it before with VS 2010 but ended up falling back to the "choose Items" solution. This is quite a pain, since I would have to do it for each project that I'm working on.