Saw this video posted on another forum where someone was trying to diagnose a similar problem (he wasn't the guy in the video).
Link to original page on YouTube.
I actually saw a Harley on the Interstate one day doing a similar wobble. It had apes on it, and you could see it oscillating the entire time. Got better under power and worse when decelerating. Didn't seem to phase the rider, who was wearing just a T-shirt and jeans - as per WI Harley Law (proper attire unless you are some kind of pussy or something...)
I bet this guy wished he was one...