With Aperture Science being rebuilt for Portal 2, GLaDOS has created a manufacturing area to create cubes and turrets for Chell's test. Just to spice things up, GLaDOS decides it would be a little bit more interesting if you just went through the place in complete darkness:
Wheatley is fortunately around to help out by shining a light on the dangerous path filled with lasers and other hazards meant to knock you into the abyss. We also get to hear Wheatley's updated voiceover at work, which is a lot more outgoing than the relaxed placeholder voice seen in earlier footage.
If you haven't heard the news yet, Portal 2 will be out February 9, 2011 for 360, PS3, and PC. At least it's a much shorter wait than expected.