What's up with the .NIB -> .XIB?

XIB's are very cool. they are basically an un-compiled NIB (in XML format if memory serves me)

during the build process they are converted to NIB's to be deployed in the final product.

As I understand it, xib files are xml nib files. xib files are compiled into nibs when the application is built. The xib format was created because nib files don't mesh well with version control (because they're not text), whereas a plain text xml file works well.

The other answers and link explain what XIB files are, but not about the specific change.

The new flat-file .xib format was introduced with Leopard (and Xcode 3.0) in 2007, but I believe that .nib remained the default, primarily for compatibility with older versions of Xcode and OS X. With Xcode 3.1, there was a shift to .xib as the preferred format. However, most documentation hasn't been updated, probably in part because the distinction is fairly minor from the standpoint of a normal developer.

I definitely echo the sentiment that XIB files are much better than NIB files, especially when using version control. (Flat files are much easier to manage than directory packages in most tools.)