Oh yeah thanks Tom.
I hugely disliked the "random geometrical hash based" avatars.
I don't know why, but really I founded it lacking in personnality.
Robots are funnier. There are also the monsters, who can be an interesting change to apply from time to time (switching between the different default proposals).
Maybe you could even allow the site users to choose in their profile what type of default avatar to use, as in the end it is just a matter of parameter in the calling url.