There are a lot of problems with the new VirtualBox 2 series. I've found many things don't work as they're supposed, I get lockups for no reason and the new features just don't warrent a full version increase. I've rolled back all my projects at work to VirtualBox 1.6.6 and decided in non-mission critical situations (home use) I'd continue using VirtualBox 2.02 and just deal with the bugs. Well, I'm officially back-pedaling on that now. VBox 2 is buggy as heck and I'm not going to continue using it.
In order to downgrade your VMs from 2.x to 1.6.6 you'll need to edit a few xml files. If you had 1.x installed and upgraded to 2.x then it will have molested your xml files so you'll need to edit them. For each VM there is an xml file in the Machines directory containing the configuration data.
/home/grant/.VirtualBox/Machines/Linux Mint/Linux Mint.xml
<GuestProperty name="/VirtualBox/GuestAdd/Vbgl/Video/SavedMode" value="1280x960x32" timestamp="1224693483507107000" flags=""/>