I don’t think that would be foolproof.
Probably not a foolproof unless an avatar generator tries to study a name for longer time period. Say it takes my name only when an activity list was considered " a stable snapshot". A method I described is just about only solution I can think of to that kind of dynamic avatars.
About the IP address use case I don’t think Riven does it, generator just returns a new image each time its reloaded. Simple and “works close enough” implementation, we really don’t pay attension to an avatar…except for now on Riven’s avatar is always under a radar.
edit:
Riven is using this URL to list an active users, this page shows a topic each user is reading atm. Combine that to a referer header and we have a pretty accurate dynamic avatar.
http://www.java-gaming.org/index.php?action=who
Using that page would probably have been better, but heck… let me give you a very good hint: the only thing I’m using is the referrer.