For all revisions of Ubuntu 12.04.* this works, at least on my Pentium D.
In order of operations:
(1) Install Synaptic Package Manager
from the control centre, if you don't already have it installed on your system.
(2) Open Synaptic Package Manager.
(3) Copy and paste ia32-libs
into the search box at the top of Synaptic Package Manager. The application will appear in a box below. Mark it for installation. Then press the apply button.
(4) Close Synaptic Package Manager when the above has been installed.
(5) Open the Software Centre.
(6) Copy and Paste ubuntu restricted extras
into the search box. When it appears, below, install it.
(7) Close the Software Centre.
(8) Download the following file (gs-esp_8.71.dfsg.1-0ubuntu5.5_all.deb)
and, when downloaded, double click it and it will open in the Software Centre. When it does, install it.
(9) Go back to the start of this thread and follow my original instructions on here for installing the Cannon LBP2900 printer.
One final thing to note:
I have been trying to migrate across to Xubuntu from Ubuntu because I by far prefer the XFCE desktop over even Gnome and most certainly over Unity. What has previously stopped me is the terrible hassle trying, more or less unsucessfully, to get my LBP2900 printer to work in Xubuntu. By doing the above in Ubuntu 12.04 and then overlying it with an installation of Xubuntu dektop, I finally have what I want; an Xubuntu installation with the my damned printer working at the same time!
I should reiterate: all of the above works on my 32 bit installation. I cannot vouch for it working on the 64 bit.