I have recently gone through this issue. I am running Lubuntu 13.04 in VirtualBox .So when I have maximized and if I would like switch to next window of main system usually we all do ALT+TAB but thats not getting for main system and going for Vbox's Lubuntu .
So how can i get activated main system ALT+TAB functionality even i have a maximized VirtaulBox operating system .
Thank you .