Installing GNOME Shell in Ubuntu 9.10 (Karmic Koala) is easy. This is how you do it.

$ sudo apt-get install gnome-shell

Once GNOME Shell is installed, you can start it by running the following command :

$ gnome-shell --replace

Remember that GNOME Shell in your repositories is a preview version and is not the final product.

If you get a black screen (as I did) , then you should set the value of the GNOME_SHELL_DISABLE_TFP variable to 1 prior to starting GNOME Shell.

$ export GNOME_SHELL_DISABLE_TFP=1