Microsoft have announced: Exam 70-540: Microsoft Windows Mobile 5.0 - Application Development
This is really great news, since it shows that MS has commitment to the platform, and sees value in people becoming certified to develop for it.
This blog entry has details how you can sign up for the beta exam:
http://blogs.msdn.com/lokeuei/archive/2006/08/21/711117.aspx
Preparation guides are a bit light on the ground, but it looks managable for anyone who has worked in this area.