I had meant to post this earlier in the week but forgot. Thanks to Kenmc for the reminder.
I went to the gents on my way out of Porterhouse Temple Bar on Sunday afternoon, where I saw this sign -- the only place it was visible:
[img:pe83hksg]http://i213.photobucket.com/albums/cc236/thebeernut/caskplain.jpg[/img:pe83hksg]
I had been sitting at the little bar right next to the cask on stillage and there was no indication that there was anything in it.
So now you know: Plain is on cask. And at room temperature. Yum yum.