Using information from Ubuntu Forums I have got video working for the T91. Here is what I did.
The video drivers are available from PPA for Ubuntu Mobile I used the information on this page to install the software sources. Make sure you select Jaunty (9.04) from the drop down box, add the sources and install the signing key as per the instructions on the above page.
Update the sources lists:
sudo apt-get update
Then install the drivers:
sudo apt-get install poulsbo-driver-2d poulsbo-driver-3d psb-firmware
Restart your T91, at this stage you may or may not have windows loading cleanly. If not, and possibly whenever you install a kernel upgrade you will have to rebuild the kernel modules for the video, this is done doing the following:
sudo apt-get install psb-kernel-source
(note that there is/was a typo on the Ubuntu Forums page where there was an s added to the above line)
Update 22 October 2009
If after an upgrade and reboot the display goes back to 800x600 try the following:
sudo apt-get remove psb-kernel-source
then do the apt-get install psb-kernel-source as above, reboot and see if that fixes the problem
I also modified my xorg.conf file as described on the above Ubuntu Forums page.
To be able to get the visual effects option to work, I needed to edit the /usr/bin/compiz script file and add psb to the WHITELIST. Around line 62
Thanks to "sammyboy405" and the "PPA for Ubuntu Mobile Team" and all those others in forums and webpages.