I'm new to using the Sforce API and I was wondering if someone knows of an example of implementing single sign-on for the self service portal. For my scenario, a user logs into our software, navigates to the portal page on our site which automatically logs them in to the self-service portal, and then the portal is displayed in an Iframe on our site.

I suppose I can do a HTTP form post to "https://ssl.salesforce.com/sserv/login.jsp" to login the self-service user but can this be done through the API instead?

I use C#

Thanks,
Jon