Russian president Dmitry Medvedev visited the San Francisco offices of Twitter and created an account on the microblogging service, @KremlinRussia. As you can see in the attached photo, the startup's founders didn't get too dressed up.
Presumably this was by prior arrangement; Medvedev showed up in jeans and no tie. Still, co-founder Biz Stone, left in the picture above, might have gone a touch dressier than a t-shirt. There are some more shots of the visit in the Twitter Flickr stream.