This weekend I took BART to Walnut Creek and rode up Mount Diablo, then back via Danville and the Iron Horse Trail. About 45 miles or so.
There are some fairly steep sections, and I had went pretty fast on the way down. I find the Birdy's handling very confidence inspiring, I was the fastest descender in the group (which also included a Moulton and a 3Rensho).
However... Above a certain speed (30 mph?) the whole rear of the bike started vibrating. It's disconcerting, but didn't seem to affect handling. I've noticed this before when I had the bike suspended and cranked the rear wheel up to high speed. I still have the wheel reflector on since I also commute on the bike after dark. I wonder if it could be enough to cause this unbalance at high speeds? I read elsewhere on BikeForums that it might - and can only imagine that the problem is worse on small wheels since they rotate more times for a given velocity.