GM is engaging its audience by publishing photos on popular photo sharing site FLICKR.
Their marketing team is pushing lots of great unique rarely seen photos to a public Flickr gallery and pulling in photos from the gearhead community too. The photos are more than "car porn" but unique and rarely seen photos covering car events, retro cards, new models and futuristic prototypes.
Other companies could share some of their corporate visual content with the outside world. The reality is that people are going to take a brand's online content anyway, so why not do it in a way which engages your fan base, encourages participation & community, and rewards good judgment & creativity?