Just a note on this: it was quite a pain to get out of seamless mode when I wanted to kill the virtual machine. First it would not accept the switch key sequence, then when I got it back, all the window decorations were gone, so no minimise, maximise and exit buttons...I'm busy playing around with seamless mode - it's quite fun having the win stuff looking as though they are linux apps rather than win apps running in a Vbox window
Also, I got a lot of black window linux apps with the Vbox running in seamless mode. This looks as though it could be fixed in 3.0.2.