I am sorry, my previous solution (uninstalling compiz) worked only on a few systems (namely only on my test pc)

and that probably was not really related to compiz itself.
I could only manage to make all of my tightvnc servers work by doing something really
ugly and dirty.
Please,
don't do that on your systems if you don't have a clear notion of what side-effects this may have. I have no clue at all about other implications, sorry. I only know that it allowed me to access my machines and still remotely use gnome. Built-in Remote Display was not a viable option for me.
So, the workaround is ugly and the only reason because I am writing these here is that I hope that it may give inspiration for someone to provide a real solution.
I launched gconf-editor, navigated to "/desktop/gnome/peripherals/keyboard/kbd" and added an invalid layout to the "layouts" key, I made sure that it was the only layout in the list. Now gnome starts smoothly on tightvncserver display, and my italian keyboard works as expected. When I launch gnome on local xfree servers it shows a couple of error box about XKB configuration errors, but apart from that, it appears to work correctly...
Please, someone provides a real fix!
