Speaking with Eurogamer, Valve's Doug Lombardi broke down their decision by saying that they don't want to "quickly replicate it [a Portal sequel] and stick it out there - do the opportunistic thing and cash in on it", and will instead be sticking to a formula where a Portal "successor has to be equally as revolutionary as the first." Lombardi went on to say that "you won't see a new Portal at retail this Christmas because of that. That's the trade off. People want more, but we don't want to give them more of the same right away because that would just be boring."
There you have it, no new Portal funnery this year. But we have a feeling a fully beefed out and revolutionary Aperture Science experience will be headed our way come 2009.