This maybe old.. I hardly check the headers of the mail. However, this morning when I click on Show details , alongside the "mailing list" : is a small URL that indicates "Filter messages from this mailing list"

when did this start ?? I don't recall having seen this before

I think this was introduced with the "Newer Version" UI.

Edit: Yep, it was:

http://blogoscoped.com/forum/112687-full.html#id114174