The Microsoft Exchange Team Blog has a great write up by Greg Taylor who is the program manager for the MCM: Exchange Server 2007 program.
The Microsoft Certified Master program is the premium credential for IT professionals and consultants working with Microsoft products such as Windows Server 2008, Exchange Server 2007, SQL Server 2008, SharePoint Server 2007, and Office Communications Server 2007.
Master certification involves a three week course (currently only held at Redmond), several written and lab exams, and a qualification lab exam. It costs $18500 (USD) and that does not include travel and accommodation. This may seem like a lot (and it is) but the program is targeted at the top 1% of IT professionals who will typically come from Microsoft partners and top-tier consultant companies.
Link: Do you have what it takes to become a Microsoft Certified Master?