For testing i’ve set up a VirtualBox, running Debian/testing in x86and amd64 architectures.
In some cases, mouse integration does not work correctly. Although mouse works perfectly in the Guest window, it’s impossible to move it out without pressing Host key (left Ctrl in MS Windows).
VirtualBox guest additions are installed, guest resizing and shared folders works perfectly.
The solution was found in VirtualBox forums :
/etc/X11/xorg.conf must contain

Section “InputDevice”
Identifier “Configured Mouse”
Driver “vboxmouse”
Option “CorePointer”

note two lines: vboxmouse tells to use mouse driver from VirtualBox guest additions, and CorePointer instructs to use this pointer as primary. Without CorePointer line the vboxmouse driver is used, but it’s still impossible to move it outside guest’s window.