[citation][nom]blazorthon[/nom]For power users, Macs lack a huge amount of software support. Even Linux is often ahead of Macs when it comes to this, although Linux can take a little work to get some things working. The problem isn't that OSX isn't a proper OS; the problem is the lack of necessary software support. For this, Windows is almost a necessity in many situations, although Macs can be used for some professional work.[/citation]
The professional software support comes from the engineers inside the company that produces a specific software, and they wont leave you out because you went with their Mac version. In my opinion you may be mixing "software support" as in "professional drivers development from Nvidia or AMD" with the support a software company like, lets say Autodesk, gives to their paying customers.
But also a Mac Pro or a Mac Book Pro are not the ultimate professional machines, they may be very solid options on specific jobs where you don't either need the ultimate workstation. So, I think you are right if we consider the "ultimate" power user and not a power user.
One clear limitation on pro Macs happens with professional GPUs. If you want a pro GPU on a Mac, basically there is only one option (a Quadro 4000) and it only supports very specific software, so you need to be aware of this.
However I just tested Maya 2013 (the current version) with a desktop GPU inside a 2010 Mac Pro and it works very well. Actually the card is officially Certified by Autodesk with only 2 or 3 features described as "working with some limitations". That is not "huge", or the "lack of necessary support".
But, if you need the ultimate 3D workstation with a Quadro 5000-6000 the Mac is not an option.
In the end I hope Apple will release new Mac Pros next week.