What we've learned is that with a 64-bit hardware accelerated gui steam will not start.
If we overwrite the 64-bit graphics drivers r600_dri.so and swrast_dri.so with their 32-bit versions, but do not re-start the gui, then steam will start.
Once steam has started, the contents browser window is blank, but there are no error messages to suggest why.
I'd say we're stuck for now..