Aside from access to SDKs, Apple developers get access to new OSs for Apple's portables and computers before the public. Now, buying your way in to the Mac Developer Program costs about the same as MobileMe.
Formerly a $500 to $3500 proposition, Apple has combined development memberships on all their platforms into one $100/year offer. For developers, that means developing for OS X is really no more expensive than developing for the iPhone. For fanboys, that means you can download the next beta OS without the need of a torrent. [Apple via 9to5Mac and Macworld]