Obviously anyone can just put http://steamcommunity.com/openid into the generic OpenID field, but a nice button would be good to make it easier for PC gamers to sign in. Nice buttons are available here: http://steamcommunity.com/dev
(Just so no one thinks I'm trying to be sneaky, I should mention that I work at Valve and am the one that just added the Sign In Through Steam buttons to that page.)