Like the E-PL1, the E-PL2 delivers noise performance on par with most similarly priced dSLRs. You can shoot pretty comfortably up through ISO 800; at ISO 1600 things start to soften and detail starts to degrade; color artifacts become readily apparent at ISO 3200.
Noise, ISO 1600
I was pleasantly surprised by the quality at ISO 1600. Though this is a little soft, the noise reduction looks very good and it doesn't clip the shadows as much as a lot of other cameras.
Olympus has tweaked its default color noise reduction, I think for the better. You lose a little sharpness, but the results look more natural. As long as you don't have small details in the image, photos shot at this sensitivity can be quite usable.
It also seems like Olympus has tweaked the metering as well. I had to manually force the settings to match the E-PL1's slight underexposure (shot about a year ago); on shutter priority, the same mode I used for the older E-PL1 shot, the E-PL2 chose a wider aperture for a brighter exposure.