No real gotchas to report, there is an issue (possibly due to the limited oomph of the pi) which causes a lot of lag after I have been using the UI for a while, if I disable Live view it settles down again.
The colours on the Canon menu don't match what is on the Live view (I can live with that), and for some bizarre reason I cant see the Print menu in the main Canon menu, pressing right and left will show the other menus, but it stays on the previous menu when it should display the print menu (this menu looks OK on the camera LCD, so the behaviour of the Live view is slightly odd). Not a show stopper as I am unlikely to ever use the print menu, but worth reporting none the less.
The bottom few lines of pixels on the Live view are corrupted in Record mode I am still trying to work out exactly why, I'll post once I have a better idea.
I get the occasional lockup requiring the battery to be removed from the camera (I haven't yet been able to reproduce this at will, so I can't give any pointers so far as debugging goes yet. I will keep you posted).
Next I am going to map an nfs mount to the pi from the laptop so I can copy the images from the UI to a folder on the laptop, to save a step.
If I get a chance tomorrow, I'll test some more.
I am most impressed with the progress so far
Thanks to reyalp, and everyone else who has been involved.