That's right. Valve has mysteriously decided to unleash several pictures upon the interwebs that feature a variety of well known Valve characters; such as Gordon Freeman, the Heavy and the Portal turret in a number of parodies of Mac adverts.
http://tinyurl.com/y96dcqq
Gizmodo article (includes all six adverts)
While there's no official announcement, we can only assume that the GDC this month is going to be pretty interesting; especially considering the Portal update last week.