Actually they do use Macs, Siemens ported NX6 to OS X at the request of Apple just so Apple could use Macs for design/engineering. NX6 requires a Mac Pro so Apple's own designers/engineers are using Mac Pros.
AutoCAD made a comeback last year and Autodesk has their Alias Design tools which has been a staple program forever. There is also SolidThinking (needs a Mac Pro), Rhino is being ported now to the Mac (needs a Mac Pro). Autodesk is really starting to have a love affair with with OS X and iOS. We run Maya (needs a Mac Pro), Nuke (needs a Mac Pro), Modo (needs a Mac Pro), Zbrush (needs a Mac Pro), RealFlow (needs a Mac Pro), Blastcode (needs a Mac Pro), Unity (runs best on a Mac Pro), etc on our Mac Pros. Weta's Mari was originally written for OS X but Apple got so behind on OpenGL that it ended up Linux with a Windows port (not as good as the Linux version), but they are probably bringing it back to OS X now that Lion has decent OpenGL support but it would need a Mac Pro because it only supports the Quadro. Sony Imageworks rendering engine Arnold has a OS X port.
Honestly a lot of high-end software outside of a shift in the mid-90s to Windows NT stayed very UNIX based and it was/is easier to port to Linux and OS X than to Windows and OS X fits into a Linux pipeline easier than Windows does.
Even the one Windows only app we run, Solidworks, we run on a Mac Pro with Bootcamp and the Windows side is just for Solidworks only, it also has to be a Mac Pro since it needs a Quadro or FireGL card. Dassault has said they are going to kill Solidworks as we know it and have hinted at a hybrid cloud system and a Mac port.
There is a lot of high-end design and CAD/CAM software on the Mac and more of the way. That is why this is the worst time to abandon the Mac Pro, it was just getting recognition again in the last few years from companies in the high-end market.
Pro audio has always needed a tower as well as color correction because you have hardware control boards.
Thunderbolt does not yet have the same I/O bandwidth as an internal bus. We are excited about thunderbolt as anyone but you can't run a video card over thunderbolt yet because you are only getting a fraction of the cards speed.
Apple would make everyone happy if they cut the handles off the box, made it a little smaller, ditched the optical drive, and come Q1 2012 put the latest Xeon chipset in there. Thunderbolt would be an option over internal storage but thunderbolt does nothing to address the power and memory needs of most high-end professionals. You can not stack minis and get a Mac Pro. You would need a cluster filesystem to completely replace HFS+ and every bit of OS X and all software that ran on it would have to be grid enabled. Apple is not doing that, even if it did you don't get more losses than procs/memory on the same bus. for a much higher costs when you consider redundant chipsets, memory, wasted video processors, etc. Thunderbolt still isn't as fast as Infiniband which is used for grid/cluster computing.
I don't know what Apple uses to design their gear, but if it's not Macs, that looks really bad, IMO. ANyone with half a brain should be asking if they don't even use their own stuff, why should we?