If ask you ask I, here's my prognosis:
As we have 3 branches of "real" UNIX
1. Linux, will progress as a humble server platform with virtually no decent desktop applications it has little chance to become a mainstream. There's almost nothing to offer for a modern computing: no real advanced photo editing, no advanced movie-video editing, no reliable office program (try forge your resume once in OpenOffice to assent to this my statement ;-) ). Linux will have profound influence on the entire UNIX community though, as all new, advanced tools come from there... As to my taste BSD still much better choice by many reasons... I think Linux will transform itself to something else or it will dye out to the OS X systems.
2. OS X had become the best UNIX ever, and the best OS in a existence. No one who has spent at least two hours in from of Tiger can go back to anything else. It unites best ever UNIX, that is BSD UNIX with stunning Aqua GUI. It Has all the commercial applications of importance running on it: MS Office (much refined than PC version), Adobe products, extensive multimedia tools, Video editing tools and so on. Great platform.
3. Big UNIXes such as Solaris and AIX (and almost dead HP) have seen better times. Solaris offers a lot but I kind of stumbled by their prices including the cost of service. All three had been the mainstream with corporative environment but gradually lose ground to Linux and OS X. Some 5-6 years ago Sun (along with ver much dead Silicon Graphics) had been a major player on graphical systems market. As far as I know this market was lost by Sun completely. AIX is one hardware platform system but for industrial environment, to my taste it is the best. It is very reliable with outstanding technical support and great, easy to use administration tools. It is possible to manage complete powerful system with sophisticated volume management after mere one week training.

Windows. I think Microsoft is in demise. It has been postponed for four years in row the release of the new version and it speaks for itself. If money hungry MS postpones release it means it implies that HUGE number of problems that it may cause massive departure from this platform. The whole conception behind Windows with deep interlinking of all embedded components made the project unmanageable. It is not only that it takes too long to publish release, it will take as much long to produce any relevant updates. I don't think Microsoft as a producer of operating systems will make it to 2010.