This didn't work for me, so after some googling I tried something else, which did work:
I opened xorg.xonf
sudo leafpad /etc/X11/xorg.conf
and swapped in (for the "Screen" section):
Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 24
SubSection "Display"
Modes "1280x1024" "1024x768" "800x600"
EndSubSection
EndSection
I had read (erroneously, apparently), that VBoxLinuxAdditions 2.10 does not support higher resolutions, but that earleir version 2.0.6 would work. It didn't.
Anyhoo... I got Crunchbang running full screen. Not new to it, really, but I've really liked kubuntu 8.10 and thought the crunchbang version is worthy of a go 'round. Something a wee bit different from kde 4.1, eh? 
Very nice work. Though the default conky must change (er, on my install)...