When Leopard was released a lot of folks (well okay, just Star Wars geeks, but nonetheless) lamented the loss of the hologram style iChat AV effect demonstrated back at WWDC. Well for those folks, a little birdy
told me that the Quartz Composer file for this effect can be downloaded here
(download link). Just pop it into your /Library/Compositions folder, restart iChat and it should be available as one of the video effects. This file was apparently taken from one of the earlier developer builds of Leopard.
If you want a different take on the same idea, there's also a fan made version over at Mac Rumors