OK...I've finally gotten it to work.
When I installed PCLOS2010 as a guest the following packages were installed by default: x11-driver-input-vboxmouse, x11-driver-input-vmmouse, x11-driver-video-vboxvideo, and dkms-vboxadditions.
I was installing Sun's VBoxGuestAdditions over these packages.
There must be an incompatibility between Sun's VBoxAdditions and PCLOS' x11 drivers or dkms-vboxadditions. (I checked my 2009.2 install and noticed that "dkms-vboxadd" was not installed by default, though the vbox "x11" drivers were).
I performed another PCLOS install from the 2010 minime .iso and uninstalled dkms-vboxadditions, x11-driver-input-vboxmouse, and x11-driver-video-vboxvideo.
I then rebooted the PCLOS virtual machine. As I expected I was greeted by a text login console (since the x-drivers had been removed). I then mounted Sun's VBoxGuestAdditions (mount /dev/cdrom /media) and installed the guest additions (./VBoxLinuxAdditions-x86.run).
I rebooted and have had no problems since.