I'm using a thinkpad W500 now. With kubuntu; I find Spaces to be practically unusable compared to true multiple desktops.
But to each their own. We each need to pick the hardware and OS combo that works for us.
Being an expert in your field is the only thing that is important.
Use whatever you require to achieve that goal, but remember they are only the tools: "It is not the tools we use which make us good, but rather how we employ them."
My Air wasn't quite cutting it for me.
Similarly, see the recent rise in virtualized dev environments (VMware, Virtual Box, etc.)
The Linux laptops with the penguin stickers are disappearing from the hackathons because its easier to decouple work from play than it ever used to be - and when you're done coding for the day, you can just fire up Chrome and surf Facebook like the other 99% of the internet.
1. The hardware and quality is second to none (personal opinion) to any PC equivalent regardless of operating system. Not even a Thinkpad which is what most PC users would often tout in my experience. Apple also leads the way in some areas of innovation here in terms of the hardware arguments.
2. The trackpad is amazing. I haven't found an equivalent elsewhere.
3. In terms of OS, Linux and Windows are both designed to be installed on almost any computer, including all Macs. You can do this by bootcamp, booting directly into the OS, or running virtual machine (what I do) and can run simutaneous OSes for various different testing and stuff without a problem. For the longest time, the opposite was not true (up until Lion) i.e. it wasn't legal to do so and even now its not very compatible. Doing a hackintosh sucks and have major drawbacks for anyone that has done this.
As a result, with a Mac, I get superior hardware, all the software options, and it runs phenomenally. And having used some Linux distro, Macs, and Windows, I can tell you I strongly prefer OS X first, Linux second, and forget Windows entirely.
There really isn't much of a competition. This is of course a vague overview. There are tons of other underlying reasons I won't get into as its too much to write about in terms of the OS and software/tools and how that adds to the development process itself which for my workflow, there are many non-comparables in either of the other two categories of OSes, especially Windows.
BONUS: You can develop for anything with a Mac, especially for the setup I laid out, but you can't say the same is true for everything else given that iOS requires a Mac.
First, the release of new Intel-based MacBook Pros around that time enabled developers to try OS X while taking Windows along with them.
Second, the new Intel MacBook Pro became the best Windows laptop, rivaling the keyboard of IBM Thinkpads but with a much better display.
Third, students got a one-time 25% discount on any Mac if they joined the Student Developer program for OS X, making prices more reasonable.
Fourth, the success of the iTunes App Store a year later made purchasing a Mac a no-brainer for iOS developers.
Fifth, hundreds of dollars of discounts and free iPod Touches have been offered to students for many years, many of whom have graduated and brought the Mac to their workplace and replaced them with new Macs.
Sixth, the release and refinement of the ultra-thin and light MacBook Air over the years culminated in affordable 2011 models that people could take with them wherever they went, such as hackathons, thus further 'advertising' the Mac to observers like you.
Seventh, the general reduction in price of Macs over the years, the improvements in the operating system like Time Machine and cheap upgrades between major releases, tie-ins with iPhone, iPad, iTunes, and iCloud, and the cool factor of owning a Mac are many factors which also contributed to increased ownership of Apple products.
Edit: from reading other responses to this question, I agree that hardware features like compatibility with Linux distros and the multi-touch trackpad have been amazing advantages of the Mac as well.
OS X provides a good trade-off between developer friendliness (because it's unix) and usability when you don't want to mess around with stuff. However, I have on more than one occasion read about developers who are pushing the limit of their development productivity get fed up with OS X and install Linux as well for when they are developing.
Also, it's Unix so if you normally work with Unix/Linux all day, then you get a great terminal environment that works similarly to your server environment. It ships with ssh, rsync, vim, etc etc and you don't have to install cygwin or worry about hitting that stupid backslash key.
That said, a lot of great hackers I know use e.g. a Thinkpad running Linux or Windows. To each their own.
If you find yourself asking why Unix over Windows, it's really because the dev tools and the command line environment it offers are much more powerful. I recommend starting with Linux. Play around with it for a few months, learn as much as possible, you'll be pleasantly surprised. The get a mac cuz it's a million times more convenient and polished.
If you take the famous ones, Ubuntu (Linux) is awesome for programming stuff and that command line makes development a lot better.
Along with the power of *nix, Mac has very good UI so many people are choosing it these days. Use what you have for now, don't spend your money buying a Mac because other people are using Macs.
I personally get a Macbook because it's a good balance -- I can run all the unix/dev tools I want, and my wife can plug in the digital camera or printer and I can plug in my iPod and have it do something sensible.
She's much happier on OSX than on FreeBSD. I'm much happier on OSX than windows.
No compelling reason not to.