Does you login window appear locked (or you get a black screen) in VNC after an user switch?
Maybe this will help...
are you using vino? the fastest solution is to switch to x11vnc :P
I've been flawlessly using x11vnc in Ubuntu for a while... but more recently I got into a problem that locked my VNC control after an apparently harmless user switching (in gdm).
The problem wasn't with x11vnc, the true origin was vino (I didn't know it, but an office mate configured it... and it was overriding my x11vnc) Note: vino is the tool initiatied by the gnome gui "screen sharing".
I don't know why (I don't have time to go deeper) but vino must have some problems to access .Xauthority or whatever it is that gives the authorization to control gdm via VNC.
Thus, I switched vino off in favor of our older and trustworthy x11vnc connection :P
Any smarter solution out there?
Config: Ubuntu 12.04, gnome & gdm