What's important here to know is that they filmed that Doctor Who episode in Utah, so of course I was expecting them being in a genuine American Diner. Furthermore, in reality you see the Cardiff bay through the glass door, but in the TV show they must have put a green screen there as you can see a highway and cars going by in the background. But then the Doctor came out of the toilet and I thought: Look, they have the same Elvis there like in Cardiff! And then it hit me that this WAS Cardiff. They had removed the Statue of Liberty and the Marilyn Monroe as that was too much.
Here are my pictures to prove it, unfortunately no whole shot to include the bar, but it is definitely the same restaurant. More pictures here: http://dieastra.livejournal.com/21151.html
Edit: The next time we were in Cardiff we went there again and now they had pictures of the TV episode on their wall ;)