I'm a video producer with a new 60D and I just did my first couple of interview shoots last week. Just some simple talking head stuff, locked off shots, and I used a small softbox and an adjustable DLC LED light. Indoors, and with just some ambient (totally overcast) daylight adding to the mix.
As I monitored the interviews in the viewfinder, I saw what looked like a subtle (but definitely noticeable) change in the lighting on the subject; I'm positive the lighting didn't change, but it had the appearance of a cloud on a windy day obscuring the sun a bit-- things would dim slowly and slightly, and then brighten back up again, slowly and slightly.
I'll check again, but I'm pretty positive I'm on "manual everything", but even if I were on auto aperture or something there wouldn't be reason for the exposure going up and down.