Found this thread:
http://communities.vmware.com/thread/333556?start=15&tstart=0
Seems VMWare Workstation Shared VMs and VPN clients (or other Winsock Providers) don't play nicely together. In my case the fix was to uninstall Microsoft UAG Endpoint Components. Rebooted, now all my shared VMs work fine :-) Hopefully this is reproducable so a fix can be incorporated into a future version.