Assuming you have the guest additions... You can get an approximate full screen across 2 monitors if you edit your xorg.conf file (I run FC11... I'm not sure if ubuntu uses xorg.conf?) and then just manually stretch the window to fill both. This definitely isnt ideal, but it does get you more visible desktop space.
I have two monitors in windows they run at 1440x900 and 1280x1024 respectively. The setup below allows a max screen size of 32000x32000.
You then just have to change the display and drag it out.
---------------/etc/X11/xorg.conf----------------------------------
Default xorg.conf for Xorg 1.5+ without PCI_TXT_IDS_PATH enabled.
#
# This file was created by VirtualBox Additions installer as it
# was unable to find any existing configuration file for X.
Section "Device"
Identifier "Configured Video Device"
Driver "vboxvideo"
EndSection
Section "Screen"
Identifier "Default Screen"
Monitor "Generic Monitor"
Device "VirtualBox graphics card"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1440x900" "1280x1024" "2720x1024"
EndSubSection
EndSection
---------------------------
1As of 4.0 Beta 2, this feature is now available to X.org clients running version 1.3+... if you are feeling adventurous. – Goyuix – 2010-12-09T23:50:20.727
2Now that 4.0.2 is out, this feature is in, right? No need to be adventurous? – Nate Parsons – 2011-02-03T19:11:01.257
I just tried it and multiple monitors works fine for me with Ubuntu 10.10 -I didn't even know Virtual Box 4.x added support for Linux guests - I gave up on checking with every release. – jmohr – 2011-02-03T20:33:00.780
@drhorrible Thanks for updating my answer, I forgot about this altogether and haven't used VirtualBox in about a year. – BinaryMisfit – 2011-02-03T20:36:43.277
No problem, I'm just coming back after learning this feature is present, very excited! This is my first step towards going to Linux as primary. – Nate Parsons – 2011-02-03T20:49:29.053
what about windows guest on osx?? – Juri – 2011-07-20T19:50:10.093