Well, i can at least ascertain that their will likely be a ep:3 some time.
I mean hl2 was released in 2004, ep:1 in 2006, ep:2 in 2008, so therefore...
Also i seriously doubt that they would ever leave the series hanging like this.
My guess is that it's source engine 2, so that for ep:3 they can do those "large scale battles" they talked so eagerly about previously.
Either that or they are indeed planning on showing ep:3, except giving the idea of a surprise leaves their options open (They could just show portal 2 or something should plans not go accordingly, like a demo map not working due to engine bugs.)
Just mah fan-boy-2-cents.