I was at the PSFK conference on Friday, sadly only for the morning session. One thing that stuck in my mind was this fantastic example of the Mii, your avatar on the Nintendo Wii and how be allowing your character to 'mingle' it will appear in the audience of other gamers.
This is a wonderful example of a relatively simple behaviour leading to much more complex behaviour (thanks to Lynette for the illustration). This technique is at the heart of the most interesting viral work. By utilising an existing platform and introducing simple rules in this way they are able to build up the potential for complex and independent social interaction - you have a life online outside of your direct control.