Not the greatest title for a post, but I want to make sure I save the next person from googling all combinations of the above words in search of how to put a sign in on an http page and have it post the credentials via HTTPS to a secure page.  I found a fakakta solution from Experts Exchange: http://www.experts-exchange.com/Programming/Programming_Languages/Dot_Net/ASP_DOT_NET/Q_21246753.html

saying to use javascript to rewrite the form action etc, but for someone like me that wants to have a sign in appear site-wide, that's not going to fly when there are other controls on the page posting back.

I give you....the postbackurl property of a button:

http://msdn2.microsoft.com/en-us/library/system.web.ui.webcontrols.button.postbackurl.aspx

It seems to be exactly what I'm looking for.  Just set the postbackurl to a different HTTPS URL.  Will it work?  I guess I'll find out shortly.