Back on 21 Nov I asked whether I could go through the US channel on holiday as I had a 5 year visa in my passport. Well, as I got to immigration at McCarran airport in Las Vegas, an official was checking people's passports. She looked at mine, saw the visa and told me to go to the US channel. I was a little apprehensive as she wasn't telling anyone else. When I got to the front, the officer just looked at me, looked at the visa, attached a white visa entry slip and told me to have a nice vacation. No electronic finger printing nor photo were taken. So I am still none the wiser but thanks to heathfield, henneth, dzug and spudqueen for your responses. I guess it just depends on the day!
Thanks for the feedback - I'm a little surprised but then at the smaller US airports they tend to be friendlier than at some of the massive ones. I suspect also there are relatively few US citizens arriving at Las Vegas on international flights, so it gives the agents on those booths something to do!