At the end of a (spoiler filled) interview with Valve team members on the Fast Company web site, Valve's marketing head Doug Lombardi states, "We will be announcing some DLC for it, so folks can keep a look out for more content coming this summer. We'll be putting more details out about that in a week or so. We'll support that with more videos or more comics." Hopefully we will see stuff that was cut from Portal 2 that was shown in the game before its release such as the Pneumatic Tubes.
Gallery: Portal 2 Screenshots