Above, shot with the actual 12.5mm F1.3 prime lens
Stanley Kubrick often used to like shooting with an 18mm lens for his trademark wide angle style, with S35mm film. The 18 is roughly 24mm in 35mm photography terms.
A more recent example is the Canon 24mm F1.4L on a 5D Mark II used to such great effect in Undercity (below). It is this kind of look I want to create with my GH2.