Is there any equivalent method in AppKit (for Cocoa on Mac OS X) that does the same thing as UIKit\'s [NSString sizeWithFont:constrainedToSize:]?
If not
EDIT: You should be able to do things the normal way in Lion and later. The problems described below have been fixed.
There is no way to accurately measure text among the current Mac OS X APIs.
There are several APIs that promise to work but don't. That's one of them; Core Text's function for this purpose is another. Some methods return results that are close but wrong; some return results that seem to mean nothing at all. I haven't filed any bugs on these yet, but when I do, I'll edit the Radar numbers into this answer. You should file a bug as well, and include your code in a small sample project.
[Apparently I have already filed the Core Text one: 8666756. It's closed as a duplicate of an unspecified other bug. For Cocoa, I filed 9022238 about the method Dave suggested, and will file two NSLayoutManager bugs tomorrow.]
This is the closest solution I've found.