I do it a lot. You need to understand the limitations of the CLS system. In a wide open space the sensor needs to be facing the trigger flash. In a more closed space, the sensor location is much less important. The bounced signal just gets it. In my last post of MMA photos in the sports section I used 2 flashes for the weigh in and face off photos. I try to make the lighting more dramatic and give some shadows to make it look poppier. I had an SB 600 with a large flash bender as a reflector up high on my tripod. It is probably about 7 ft tall like this. I shape the flashbender into a cupped shape to sort of scoop the light forward. The other flash was an SB900 on camera turned way down to provide a little frontal fill. This setup works great but I fall back on my ISO power of the D3s to make it work well. I shot at ISO 5000 for this one I think? Maybe 3200? It lets me use my camera as the main background fill and just use the flash for pop. It is quick and dirty as it gets.

The nikon system with the pop up works amazing too. Some of my favorite portraits I have shot have been with my 300s pop up either just triggering or doing very minimal fill and hand holding a flash to the side to provide some serious sidelight. This is a situation where a 3rd arm would be great.