We don't know what kind of downloadable content Valve is planning for Portal 2 (that should change soon), but we do know it's coming this summer, according to a mildly spoilerish Fast Company interview.
"We will be announcing some DLC for it, so folks can keep a look out for more content coming this summer," said Valve VP of marketing Doug Lombardi. "We'll be putting more details out about that in a week or so. We'll support that with more videos or more comics."
There was some downloadable content released for the original Portal, a collection of additional test chambers created by the fan community. The stages were later bundled together, labeled Still Alive, and released via Xbox Live Arcade. Still Alive did not feature more Portal story, either, just more puzzles. It's unclear whether Valve intends to find a narrative hook for Portal 2's DLC.
(But I hope so.)