Jur--The "low frequency shimmy" does not sound like shimmy at all, but rather a dangerous speed wobble. In the hands of a less capable rider, that can cause some big problems. This is not a known problem with the Birdy from what I've read on the forums. But Martin Donnely on the Yahoo groups forum experienced speed wobble on a 1990s era Birdy that was heavily modified (and cracked). You might want to contact your dealer and try to get Pacific/R&M to replace the frame for you. That would be totally unacceptable in my book. Unless I'm in sand or powder, I find both of mine to handle well at speed.
Are you certain that there isn't anything pressing against the moving arm of the front suspension?