Here are some links to articles and google groups discussions about how to attach a cert to an aspx page request, as well as how to extract and inspect it on the receiving side.

Google groups discussion


Building Secure ASP.NET apps: authentication, authorization and secure communication.
Go up to chapter here on MSDN for more about security on ASP.NET pages.

Answer from MS support about programatic access to client certs.

(the broken link to the isaserver.com article is here)

(see overall WSE 2.0 topic below for more general WSE info)