Woah- this is a lifesaver. Fantabulous. Makes Parallels work again for me.
I was experiencing this same problem (enter full screen in Windows XP, do work in XP, hit the default hot key out (command-enter, or command-alt-enter...) and it flips and ~poof~ parallels crashes with a SIG BUS error. It drove me crazy to the point that I reinstalled OSX from scratch, installed Parallels, and then installed a brand new Windows XP install. Still crashed! (Happens on my MacBook Pro 17" and my MacBook Pro 15").
I submitted a ticket to Parallels tech support last week and got an automated "thanks for sending this" message but no follow up or solutions.
So needless to say, this June 14 fix was superb.... Not sure of others are having this problem. but this does the trick. Remap the full screen hot keys to not include Command and you're set.
woot!