In opposition to the article I called brilliant, earlier, here's what appears to be the OP's response...

ongoing: Open-Sourcing Apple Apps:
And remember, if Apple gets the licensing right, they’d be free to grab the good hacks & patches that the community comes up with, and this would help them keep up in the UI race with Windows on one hand and GNU/Linux on the other.

Sorry, but I'm not buying. First, I certainly hope Apple has more know-how in the QA department than to let user-crafted hacks into the OS distribution (that is, the OS and its stock apps, like Mail.app, Safari, etc). Incorporating end-user hacks and patches into a desktop-consumer, user-friendly, tightly-controlled package (eg, the Macintosh operating system) is simply not worth the QA work. I can't imagine -- one of the most important things to keep constant in a software development house is coding style, a coding rhetoric. I've yet to be impressed with the coding rhetorics of an open-source project which is not either one-coder or incredibly tightly controlled by one maintainer. With the latter, I'm often unimpressed with how "not Open Source" contributing to the project feels.

For the most part, I'm afraid Apple's interactions with open source are going to continue like they have to this point. They'll take large, mature, useful codebases like FreeBSD & Konq, spend their time cleaning these apps up (Apple-ifying them), which includes having them pass internal QA, and then occasionally return small favors to the open source commune when the opportunity presents itself. I'm personally not a big fan of that approach, but it's a sound business plan. If you don't want people stealing your large apps, don't license them with BSD-style licenses. If you like GNU, talk to the BSD folk and try to convince them to stop allowing such "robberies" to take place. If you use BSD, congratulate Apple for taking advantage of your contributions correctly as a corporation whose birth was enabled by and whose existence remains fueled by capitalism.

The kicker from the post is the idea that user-created hacks could benefit Apple on the UI front compared to Windows and Linux, even the idea that Apple has work to do in that department. With Darwin underneath, which is an indescribably gigantimongous improvement over the Classic kernel, good UI really still is Apple's primary claim to fame. Haven't we seen the new Apple commercial about iTunes -- the Mac's got iLife and Windows has Clock? That you can plug the fancy camera into a Mac out of the box? Interfacing *is* Apple's gig. And, um, what desktop app UI is so wonderful in GNU/Linux again?

Who in the world would champion using user-provided hacks as a way of helping a company -- a company whose best gig *is* interfacing -- surpass an OS more famous for its use on servers than desktops?

From the post:
I work at Sun Microsystems. The opinions expressed here are my own, and neither Sun nor any other party necessarily agrees with them.

Sun may not necessarily agree with him, but it looks like the corporation's culture is meshing right well.

(In case this is too oblique a reference, please search this blog for Swing and AWT rants. Why did Java have a UI at all after they'd lost the war (skirmish?) for the desktop in the 90s? This is a rhetorical question, but the answer involves JDBC. Sun [seriously] does a great job incorporating open source server-side apps into their business model, but Solaris didn't exactly win the desktop either.

Before you get too upset, this post, what you're reading now, is from a guy who writes desktop apps in Java as a side-business. From a development standpoint, Windows has won the UI battle. From a user standpoint, Apple's developers have, in many arenas, overcome their initial deficit with UI development tools and won the practical UI battle. Windows has the potential, Apple has won the war in practice. (Unf. for Apple, the UI war is not the end-all of software design!))

If there's any middle ground, it would be to open source the internals of the applications, well stubbed to allow laying any UI on top one would like. Mail.app's UI would remain exclusively an Apple creation. I still don't like the idea (and neither does the OP, afaict), and think it's nearly impossible to provide a well-tested, Apple-like interface without quality in-house testing of each code submission (which includes knowing, even picking the coders), but internals are where open source has much better credentials.