You can do Cocoa development in any editor you wish to use. I edit code in TextMate, and only use Xcode to manage the project overall, open the xibs in Interface Builder, etc. It would be easily possible for anyone else to build an IDE for developing with Cocoa. The problem is that Xcode doesn't suck too bad, and it's free, so it would be difficult to sell another IDE to support the development costs. Several years ago, Codewarrior supported Cocoa development. As Xcode was improved more and more, less and less people bought and upgraded CW and therefore it eventually went away, unable to compete with Apple's free offering.
Cocoa also doesn't lock you into following strict MVC. In fact, it's even easier than Rails to merge model logic into the controller. It's also even easier than Rails to end up with what should be view code in the controller. In fact, simple apps may end up almost entirely contained within a single controller if you're not careful.
Jim
