No, there are not any browsers that support this, but apparently it is not very complicated to implement server-requested TLS upgrades in Firefox or any another application using NSS:
http://archive.netbsd.se/?ml=mozilla-crypto&a=2005-10&t=1393140
The way I see it, Firefox would look for responses with HTTP 426 (Upgrade Required) code and then the exchange specified in RFC2817 would take place. I am not sure this will actually work :) One step at a time... :) Apparently, it has been done before, but not documented.
Peter
