Well, this is new. I don't know if they rolled it out with 10.4.3 or when it came out, but if you have an iSight or other webcam plugged into your Mac and you use iChat to take a video snapshot of yourself, the entire screen flashes white momentarily just as does Photobooth. I remember seeing Steve Jobs' demo of the flash effect during the iMac/Photobooth announcements, but I hadn't noticed it being something that they were pushing out to all Macs via iChat.
In any case, if you want to test it out yourself, plug in a Firewire Webcam, launch iChat, and click on your picture in the top of the iChat window. A drop down menu with a list of recent pictures will pop up, and at the bottom of that list will be a button that reads "Edit Picture..." Click on that button, and at the bottom of that frame click the Take Video Snapshot icon. Notice how it beeps with the same little red countdown that can be seen in Scott's video podcast of Photobooth and then blinds you with a flashing screen of white light!