Seeing as it took me a while to find the official firmware updates for the Ampe A90,
I thought it might help other people if I posted the following link.
The latest firmware currently for the A90 (Single Core - ICS 4.0.4 from 27/08/2012) can be found here.
The latest firmware currently for the A90 (Dual Core - ICS 4.0.4 from 28/09/2012) can be found here.
I have created a quick guide on how to upgrade here