Open a terminal and type following command to enable the remote desktop:

$ gconftool-2 -s -t bool /desktop/gnome/remote_access/enabled true

To disable the “Ask for Confirmation” dialog

$ gconftool-2 -s -t bool /desktop/gnome/remote_access/prompt_enabled true

If you check

System -> Preferences -> Remote Desktop

you will see that it is now turned ON.
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