I have passed 70-442, which is the equivalent exam under SQL Server 2005 (instead of 2008). I didn't use any preparation materials, just went ahead and took the exam. I have taken this and several other "PRO" exams, and I have found that (contrary to the
"TS" exams), the questions in PRO exams can be answered by aplying logic, experience, and general knowledge about the platforms and technologies involved.
For the 70-451, Microsoft suggests the course
50401A: Designing and Optimizing Database Solutions with Microsoft SQL Server 2008. I have never taken a look at this one, so I can't comment as to how much it will
help to prepare for the topics of the exam.