Are you using Tab or floating windows?
I am using the appplication frame with tabs on a mac.
Omg, today I just ran into this problem too.
At first I thought it couldn't open the file because the canvas seems like pixel small. I had someone with CS6 photoshop to open at her mac, and it opens fine.
So I opened the file again, and saw the viewing percentage at .8 percent. I made it 100% at where it says ".8%." and the application frame gets back to normal. I think it's the application frame that's causing it, so I turned it off. Now the file opens fine.
Also, it could be the file that's causing this problem. It's a CS4 file that I was trying to open with CS6.