Every time I create a new document, no matter what dimensions I put, it always makes it a square. Just tonight it has. I've been using the program for almost a year now and havent had this issue at all. I don't think I messed with any settings or anything. I read somewhere that it might have to do with the Pixel Aspect Ratio or something, but I've been trying different things with that and it hasn't worked.
I'm on Mac OS X 10.8.2 Mountain Lion running Photoshop CS5
Could you please post a screenshot with the Image Size dialog open to illustrate the issue?
It shows the dimensions correctly but it's definitely not right. I've tried lots of different dimensions and none work, it's always the same.
The screenshot looks fine here (and the canvas seems to have the proper diagonal for 1280x1080).
Please post images on this site directly in the future.