Roughly 6 months after Apple released the first Retina Mac, Adobe has updated Photoshop to make use of every one of those double-density pixels (aka HiDPI). Adobe was expected to go Retina by the end of the year, and they made it with weeks to spare.
Illustrator and some of the other CS6 apps should be going Retina as well shortly.
If you subscribe to Adobe Creative Cloud, or if you own a Photoshop CS6 license, head on over to their software update tool and give it check. Your eyes will thank you.