Well, I'm catching up to this thread a few days down the road, but I wanted to respond to the idea that Av was forcing the camera to the synch speed.
Tonight I put a flash on my camera with the power off, and set the camera to Av mode. I took a shot and it exposed for the ambient. I turned the flash on and took another shot and the shutter speed stayed the same. In other words, in Av, the camera exposed for the ambient, as I said. I prefer to do this type of shooting in Manual to control this, but I thought you should know.
Jeffrey, try it out and if you get different results, let me know, maybe our particulars differ.