The pre-installed VBox guest additions in PCLinuxOS 2010 doesn't work with VBox auto-resize guest display (With PCLinuxOS as guest)
Removing the pre-installed ones via synaptic and installing new ones manually makes this work.
But I also encountered another problem with it.
After every boot PCLinuxOS returns to the last auto-resized resolution instead of one manually set via the KDE display settings.
(It also causes some strange resizing behivor with the desktop image, it gets stretched)