I use Remote login with vnc-ltsp-config with GNOME Desktop Environment on CentOS 5.9. From experimenting today, I managed to get cut and paste working for the session and the login prompt (because I'm lazy and would rather copy and paste difficult passwords).
I created a file vncconfig.desktop in the /etc/xdg/autostart directory which enabled cut and paste during the session after login. The vncconfig process is run as the logged in user.
[Desktop Entry]
Name=No name
Encoding=UTF-8
Version=1.0
Exec=vncconfig -nowin
X-GNOME-Autostart-enabled=true
Added vncconfig -nowin & to the bottom of the file /etc/gdm/Init/Desktop which enabled cut and paste in the session during login but terminates after login. The vncconfig process is run as root.
Adding vncconfig -nowin & to the bottom of the file /etc/gdm/PostLogin/Desktop also enabled cut and paste during the session after login. The vncconfig process is run as root however.