MSPX is a custom extension Microsoft uses for their content
presentation framework:
http://www.microsoft.com/backstage/bkst_column_46.mspx
"The presentation framework includes a custom Web handler built in
ASP.NET. Pages that use the presentation framework have the .mspx
filename extension, which is registered in Microsoft Internet
Information Services (IIS) on the Web servers. When one of the
Microsoft.com Web servers receives a request for an .mspx page, this
custom Web handler intercepts that call and passes it to the framework
for processing."
- Jon
http://weblogs.asp.net/jgalloway