I run Ubuntu Desktop in a VirtualBox seamless mode, hosted on Windows 7. I recently upgraded from 11.10 to 12.04 and seamless mode no longer works correctly. I have re-installed guest extensions.
The problem is that the Ubuntu desktop wallpaper is shown, effectively blocking the Windows desktop. I can bring Windows apps to the foreground, but as soon as I bring any Ubuntu window to the foreground, all the windows apps will disappear, hidden by the ubuntu desktop even if I activated an Ubuntu window in a different monitor. So, it is not very much different from full-screen mode at this time.
I suspect if I could turn off the Ubuntu desktop or wallpaper this would be a sufficient workaround (this is basically what Seamless mode is) but that does not seem to be possible within Ubuntu itself.
[Jeremy doesn't specify system config, but I can confirm I'm seeing this behavior in Win 7 64-bit with both 32-bit and 64-bit guests. Giovanni sees the same thing on his laptop running Vista Ultimate 64 bit after upgrading his 32-bit guest to 12.04 (forums.virtualbox.org p225407, requires logging in with your free Oracle account). I was using vbox 4.1.14r77440 and matching Guest Additions. I upgraded to 4.1.16-78094 vbox+additions. Still a problem. And I filed launchpad bug 999929 to see if I can get feedback from developers. – tbc0]
Any other ideas?