what could a Web2.0 virus look like?
How could a virus like this be damaging? - it could delete your mails, your documents, contacts, etc. - it could do some URL rewriting to add a specific associate id to links to amazon and other on-line stores. - it could launch a distributed DoS attack agains a site...
Replace Google with Microsoft Live above to get a more destructive effect.
I'm pretty sure this will happen sooner or later. And then we will start looking for the Open Web2.0 platform that has fewer virusses, etc... :-)