Gnome 3 fallback not working with Silicon Motion GPU

[Andrew Wood <andrew@perpetualmotion.co.uk>]

Im trying to get Wheezy with Gnome 3 in fallback mode to work on an old 
machine which has a Silicon Motion SM712 LynxEM+ GPU.

It starts up in fallback mode OK but the display isnt right. The theme 
for buttons & scrollbars is just the generic GTK look i.e no theme at 
all, (but the window border & close/max/minimise controls have the 
correct Gnome 3 look), certain widgets on the desktop like the clock & 
sound icon display as white rectangles and the terminal window, instead 
of being a white background with black text displays as all black.

The xserver-xorg-video-siliconmotion package is installed but I dont 
know if its actually using it. How can I check?

Thanks
Andrew