I'm running Ubuntu 12.04 32 bit as guest on Windows 7 64 bit. Everything is fine except one thing..when take my mouse pointer near the top edge(I set it at top) can't get the floating menu of virtual box in full screen mode...so I'm not able to minimize my Ubuntu virtual machine. Even worse the ctrl,alt+tab combination to switch out of windows is also not working.
I'm also running Linux Mint 13 Mate as guset where everything is fine so i guess this an Ubuntu problem may be related to compiz etc stuff.
Virtual Box version : 4.2.6
EDIT: guest additions are installed. Before installing additions Ubuntu was running in 1024x768 and now in full wide screen. As far as I remember the floating menu was available when guest additions were not installed.