Oracle released VirtualBox 3.1.8, a maintenance release of VirtualBox 3.1 which improves stability and fixes regressions. Among other fixes it contains support for Ubuntu Lucid Lynx hosts.
See the Changelog at:

http://www.virtualbox.org/wiki/Changelog

for a complete list of all changes. You can download the binaries here:

http://download.virtualbox.org/virtualbox/3.1.8/