There are only 2 games I can think of that I know Valve is developing, can be to the point of showcasing, and they would be willing to bump Portal 2 to their booth for: CS 2 and HL 3. This should be interesting unless they're doing Bejeweled Source.
The memo promises, "the event will be replaced by a surprise," adding, "the cancellation of the event is not THE surprise," and furthermore insisting, "the time, date and content of the actual surprise will only become available as you experience the surprise."
http://www.joystiq.com/tag/portal-2
