As said, VirtualBox doesn't use the system libraries and so looks quite aweful (at least in a Gnome environment). This patch for the ebuild creates symlinks during the install phase to the system libs.
Steps to Reproduce:
Created attachment 234511 [details, diff]
Patch for the ebuild
Sorry to disappoint you but we cannot make a binary package use system qt libs. It may work now but may fail as soon as we for example get qt-4.7 into portage.
If you really find virtualbox-bin looking so ugly you may switch to virtualbox-ose which links against sytem qt.
*** Bug 319271 has been marked as a duplicate of this bug. ***
*** Bug 326711 has been marked as a duplicate of this bug. ***