According to the website the offer is valid until 30 June 2010: http://www.prometric.com/microsoft/ss_mcp.htm for terms and conditions.
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No dates as yet for upgrade exams, but if you're an existing MCPD (Windows, Web or Enterprise) you'll be able to take a single upgrade exam (except for Enterprise where 2 exams are required.
http://www.microsoft.com/learning/mcp/mcpd/vstudio/2008/default.mspx
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