By default OpenBSD randomizes the ports used. The same randomization is done by
djbdns.
The CERT advisory
http://www.kb.cert.org/vuls/id/800113 contains three links to Daniel J. Bernstein web pages about DJBDNS and he is credited for the original idea of using randomized source ports.
BTW according to
http://cr.yp.to/djbdns/forgery.html Bernstein predicted this issue already in 2001