I have 2 displays: laptop @ 1600x900, external @ 1680x1050
When I put a VirtualBox (Win 7 guest) into fullscreen or seamless mode on my first monitor (laptop), it works correctly.
When I do this on the second monitor, it only expands the guest to 1600x900 and not the entire 1680x1050 screen.
The windows guest sees this (1600x900) as its maximum resolution so I cannot manually increase it from there.
I used to do this a lot, and it has worked in the past. The most recent update I have made seems to have broken this functionality. I recently updated to 4.0, but I'm not sure what version I had prior to that. I believe it was 3.2.something.
When in windowed mode I can actually resize the guest to larger than 1600x900, but when I then hit Host+L or Host+F it scales it back down to 1600x900.