What could a web2.0 virul look like? Here is my guess:

• it will be written in Javascript
• it will live inside Google Documents
• when you open the infected document, the js will look up your Google contacts, create an infected document and invite all your contacts to collaborate on it.
• when one of the persons invited opens the infected document, the virus will replicate, and so on.

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... :-)