Apple offers a number of certifications, but none in programming:
http://training.apple.com/certification
I would definitely say that if you're seeking employment, having an example application that shows mastery of Cocoa (obviously you'll show a mastery of a subset, not everything), and some concepts like Core Data, etc. will take you further than a certification. I may simply not be seeking these types of jobs, but I've never seen MCTS/MCPD/MCAD/MCSD required for a Windows programming job... most certifications i know of are geared towards the IT side of things.
In any event, learn to program OS X and earn the skills you might imagine would be needed to get a certification and you should be fine. Actually doing whatever task a certification is meant to prove you can do is better in my opinion than the certification itself, which can be come by surreptitiously.
-Lee