After the Windows Phone 7 exam 70-599 was officially published on July 14th, Microsoft also added information about the certification requirements which was missing from the MCPD site.
This finally clarifies that you indeed need exams 70-506 (Silverlight 4 Development) and 70-516 (Accessing Data with Microsoft .NET Framework 4) in addition to 70-599 to receive the MCPD certification.
Another important change is that unlike most other MCPD certificates, this one is only valid for a limited period of time and you need to re-take the exam to prove your ongoing knowledge every two years.
More information about the certification requirements can be found at Microsoft Learning site.
P.S. Information about this update has been also added to Peter Kuhn's "Getting ready for the Windows Phone 7 Exam 70-599" series.