VS 2012
The best answer I have found so far is to drag the Code Analysis view’s tab away from the Solution explorer. I’ve docked mine below the Solution explorer and made it about 1 inch high – so I can still see what it is reporting on the rare occasions when I wish to check it, but without losing my Solution Explorer all the time.
It’s still unnecessary but a lot less irritating.
Update – VS 2013
In VS 2013 you can now click the Settings option in the Code Analysis view, and disable the Show window when issues are detected option.