Yes gener7 is spot on. Photoshop does not play nicely with two GPUs. If you follow section 7 in the link he posted you should be OK
Thanks for your help gener7 and davescm - I disabled SLI and restarted photoshop and it was able to detect the GPU.
If I re-enable SLI, it won't detect it.
Its the only application that has a problem - all my other apps work fine with SLI enabled.
Being a game developer, this is a bit of an inconvienience.
In the NVIDIA profile, I set it to Single-GPU for both sniffer and photoshop - but this doesn't work either.
I also tried renaming sniffer after turning sli back on in the hope that it would keep the settings from before where it had detected a graphics card, but it seems photoshop does graphics card detection, and therefore still won't work.