The virtual world of Second Life has recently been suffering from a series of attacks from what has been referred to as "grey goo", a term which is a direct reference to the scenario of uncontrolled exponential growth in nanotech replicators. The result of a grey goo attack is that the world fills up with junk that prevents anyone getting anything else done.
I haven't covered this before because it is well known to the point of infuriation to most people connected with Second Life. What's been more interesting recently is that people outside that community have started picking up issues like this from Second Life, particularly people more commonly associated with security in general. For example, Ed Felten wrote a couple of articles recently about the "copybot", which allows you to make a copy of anything you can see in-world without paying for it (with some limitations, which aren't relevant to this discussion). Professor Felten is perhaps most well known for his work on the SDMI challenge, US v. Microsoft and more recently the (in-)security of electronic voting machines.
Directly on point to the grey goo attacks is Eric Rescorla's Beta-testing the nanotech revolution; again, this is a bit off what most people would think of as Eric's normal beat.
But that's my point: if you're involved however peripherally in security systems, you walk into something like Second Life and see a lot of problems waiting to happen; as Ed Felten puts it, these are really issues "from the It-Was-Only-a-Matter-of-Time file". New systems should be learning from the mistakes of the past, not blundering through a series of unworkable solutions every time until they get to something that works until the next bad guy comes along. Unfortunately, that doesn't seem to be how the world operates. Ed Felten has another appropriate quote for this: "Given a choice between dancing pigs and security, users will pick dancing pigs every time."
If you're interested in a bit more comment about the grey goo problem per se, I attach the comment I added to Eric Rescorla's article below.