I'm running Ubuntu Lucid (10.4) x86_64 as a guest system on a headless Debian Lenny Xen hypervisor. On the Ubuntu machine, I run an X-server with a normal Gnome Desktop which is only reachable through VNC (using vnc4server).
On this X-server, every kind of drag & drop interaction seems to be disabled, e.g. moving Desktop icons around, adding icons to the Gnome bars from the main menu, reordering Firefox bookmarks, dragging text from one text field to another.
If you have an idea what could cause this, please tell me.