Note to self - if this happens restore the pre-installation vmware_drv.so as per this link:
and this one too http://knowledgefrontier.blogspot.com/2008/04/opensolaris-snv86-on-vmware-with.html
which is essentially to do this:
mv /usr/X11/lib/modules/drivers/vmware_drv.so.BeforeVMwareToolsInstall /usr/X11/lib/modules/drivers/vmware_drv.so
luckily vmware keeps the old copy, which is handy....
this is dead easy when you know it. When it only shows up as a problem after a reboot, and the time between installing the tools and rebooting is days or weeks with many other changes in-between - working out the cause and effect relationship is not always so obvious :)