HSTS is a security policy mechanism where a Web server tells a supporting browser that it can only connect to it over secure connections (i.e., SSL). HSTS allows website operators to force secure connections with users that are using an HSTS-supporting browser. Supporting browsers include: Google Chrome 4+, Firefox 4+ and Opera 12.
I have spoken about HSTS before in a previous blog post and will update again once it has been finalized.
Updated November 21, 2012: HSTS has been published as RFC 6797.