How can the alert system be triggered? Who can do it? Since we established the alert system is included in the official client, I would like to know who can trigger the alert system, and how?

Can this system be spammed (activated 1000s times per second) thus disabling any use of the client? (Because he would get annoying popups all the time)
Just got confirmation that Satoshi passed the signing key to Gavin before he retired, so at present only Gavin (and Satoshi if he ever returns) is able to send out alerts.