I'm just beginning to work with Flash video export in Premier Pro CS3.
I don't see any option for selecting CBR or VBR output.
Does this mean that it is CBR? CBR by default? CBR only?
I don't have CS3 installed any more, but I seem to remember that there was the option for VBR encoding, along with CBR. I'm working from memory on that, so this memory is subject to error However, it was 1-pass only. Two-pass VBR encoding--at least for VP6--was added in CS4. The VBR option should be somewhere on the Video tab in the exporter window.