I'm running PCLinuxOS in VirtualBox for test purposes. I decided to see how smoothly the upgrade from KDE 3.5 to KDE 4 would go.
Good luck Mark. I hope that more VBox experts will be along soon to advise.
In recent months I have been doing just as you have with a virtual install of PCLinuxOS 2009.2 which I have updated to KDE 4, but using VMware Server. The basic install and upgrade went very smoothly indeed - couldn't have been easier, in fact - so it might be worth considering this if your problems in VirtualBox persist. (Since recent updates I am now having problems myself getting VMware Server to run properly in PCLinuxOS, but this may be partly my fault for attempting to go on running an 'old' version in the 1.0.x series ...).
I'm having trouble with VMWare Server, as well. I believe it is because the latest kernels were compiled with GCC 4.1, and PCLOS is now using GCC 4.4

Haven't had the time to try recompiling, so I'm just waiting on the newest ISO timeframe, in the hopes that the complete toolchain upgrade will include a newly compiled kernel...VirtualBox works fine, in the meantime (and, they can both be install on the same system, and run at the same time ... as long as they don't both try to use HW virt support in the CPU).
I had a VirtualBox MiniME that I upgrade to KDE4 for testing a while ago...it went fine. I could probably remove it now, as I went ahead and did it on the physical hardware...but I'll probably keep playing with it until I'm dying for disk space (again), and remove it then,
