vim /etc/apt/sources.list
deb squeeze contrib non-free
wget -q -O- | apt-key add -
aptitude update
aptitude install virtualbox-4.1
vboxwebsrv -V
VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.1.*.vbox-extpack

VBoxManage extpack install --replace Oracle......

Если возникнет ошибка вида
The following NEW packages will be installed:
libpython2.6{a} libqt4-dbus{a} libqt4-network{a} libqt4-opengl{a} libqt4-xml{a} qdbus{a} virtualbox-4.1{b}
The following packages are RECOMMENDED but will NOT be installed:
libhal1 libsdl-ttf2.0-0 python-central
0 packages upgraded, 7 newly installed, 0 to remove and 0 not upgraded.
Need to get 74.4 MB of archives. After unpacking 145 MB will be used.
The following packages have unmet dependencies:
virtualbox-4.1 : Depends: libssl0.9.8 (>= 0.9.8m-1) which is a virtual package.
The following actions will resolve these dependencies:

Keep the following packages at their current version:
1) virtualbox-4.1 [Not Installed]

Accept this solution? [Y/n/q/?] n

То решается установкой библиотеки
dpkg -i libssl0.9.8.deb

Дальше все делается так же как здесь