I hosted some CouchSurfers a while back, and showed them around the CBD. We wandered around the famous laneways, and took lots of photos of the graffiti. Check out my dphoto album.

Also, a quick Google Search found these graffiti links:

• SHIFT | PLACE | MELBOURNE – GRAFFITI ART, an excellent summary of the scene, with some great photos.
• There are also some photo examples in this photo gallery on About.com
• The writing on the wall | Guardian Unlimited Arts has scathing political commentary about the state of the art.

Enjoy!