I took a Jetstream P8 out for a quick test and was impressed with the ride. However, there are no Birdy's around for me to try (not until next spring, says the one LBS that sometimes carries them), so I can't comment on a comparison. You can compare "the specs" for current and older Dahons right on their site.

2006: http://www.dahon.com/archive/2006/us/jetstreamp8.htm
2007: http://www.dahon.com/us/jetstreamp8.htm

(P.S.: Those are the U.S. models -- the International models are pretty easy to find, though)