The world is full of prejudices. All designers use Apple Macs is one of them. This was surely true 10 or 15 years ago, but todays designer are well distributed across all platforms. There have been several reasons why Apple Computer where so dominant at that time, but all of the reasons have become obsolete and you can choose yourself if where you want to put your money - into looks or content.
The operating system wars are over - no really - OSX and Windows 7 are equal in functionality and ease of use. They are actually so similar, that one often gets confused where to find specific things when switching from one to another.
Availability of professional and free software for designers on the other hand are actually not the same. Unlike in the early ages Adobe and other developers have switched to PC and Windows as their main platform. Many features (GPU acceleration) and even whole Suites of Programs are PC only.
And if you are a Linux fan, even then you have plenty of open source software your can use and work pretty well.
One of the biggest game changer in this sector is the internet. If you design websites you are used to use "development" tools. And more often than not this tools are easier to find for other platforms than Mac. It's the "App"-Numbers Game you see on Iphone vs. Android/Symbian/WebOS/Blackberry - but inverted. Hundreds of applications for Mac, thousands for Windows and Linux.
Another big game changer was actually PDF. The designer doesn't have to use the same Programs and computer platform as the printshop, since PDF has become the standard for sending printable files. It's a typical example of how a "locked in" environment has been broken by the use of open standards.
Apple hardware looks good. I'm not going to deny that. Until last year the apple fans could also claim, that the hardware inside the boxes was always top notch and not more expensive than PCs if you take the same components. I made the calculations myself a few times and it was true. Mac Pro's used processors and chipsets which you find in serious workstations. Picewise obviously if you compare a Mac Pro with a standard Dell Office PC, the Dell wins - but the performance is not the same.
But here is the news - you don't need the power of a Mac Pro. Actually most designer (if they are on a Mac) use the Macbooks or iMacs. And even the Dell Computer intended for the use in the office runs Indesign and Photoshop smoothly. The recent office PCs have more performance than a workstation 3 years ago. The possibilities that your computer is waiting for input 90% of the time is high if you are not encoding HD-Video all the time.
So there is for the average designer no need to buy an expensive Mac Workstation to do the work. And if you really need the horsepower? Well than you have another reason to not choose an Apple.
Apple is not upgrading it's Macintoshes
Normally we have seen that Apple is a Computer and Hardware Manufacturer who does extra stuff to sell more hardware. This is changing and we can see it every day. Apple is focusing on "consumer stuff" and content related stuff.
It's more about iTunes, iPad and iPhones this days than OS X and Mac Pros. In the past Apple was the first to implement faster processors - now they don't care. They prefer to release a new tablet instead. Today if you want a serious workstation you get an HP or even a Dell. The lack of new announcements for Macbooks etc. show in my opinion that they stop to offer hardware which runs an relatively open operating system altogether. And sincerely - as professional I will not invest in software and hardware for a platform which future is not certain.