Virtualbox is a great tool, BUT
VirtualBox 4.0.x hangs my host OS for a minute or so every time I start (or resume) a VM. 3.x.x worked fine.
The host is a Windows 2003 x64 Server (64 bit AMD host). One of the guest OS is 64 bit Ubuntu 10.04 LTS Desktop. This only happens when nested paging is enabled.

I tried to post a report about this on the virtualbox forum, but I had some difficulties with the registration page. I kept getting the ‘The field “VBox Version” must be completed.’ error message when I type in the correct ‘4.0.8’. When I try 3.2.12, or something else I get an additional line: ‘Enter the Current VirtualBox Version number in the form 9.9.9 (See our website for the latest Product Release Announcement)’ besides ‘The field “VBox Version” must be completed.’ It turns out the missing field was the license type. I guess you could call it Version, but why would you?