Partly because dhcpserver on hostonly network connection was not working on the previous version.I installed the latest version over my working installation. Then I made a mistake & installed the addon for the previous version, correcting this to the latest version
Result was none of my virtual machines would start. Reinstalling with the repair option did not correct the situation.
I found that all of my Virtual machines had been hopelessly corrupted.
now I have got the latest version of VirtualBox working OK, & ending up reinstalling a virtual machine for Ubuntu 11.04 & Linux Mint 11.04, which appear to work OK now.
However Open Suse 11.4 during & after installation was very slow & I have not found out why. It was AOK on the previous version
Further comments are that I think it might have been pointed out that installing a new version might well wreck your virtual machine, and that you must be very careful to install the correct version of the VirtualBox Extension Pack
The other thing about the new version of VirtualBox is that I can no longer switch to terminal 1 & see the end results of the bootup sequence:- just a messup of the Plymouth image.
One other thing I find awkward is that to start the dhcpserver for hostonly networking one has to use the awkward command-line driven VBoxManage I would imaging it should not be too difficult to make up a graphical interface to do this. But One cannot really ask in respect of Open Source Software.
I found out one other thing:- Proftp server on Linux does not like the Host Name being tidied up, which results in it failing to start.