Panzrwagn is correct about the OS "Winblows vs *nix," a business could not care in the least which operating system is running an application.
Individuals have a favorite, right or wrong about which OS is best and that is generally what they run at home or develop freeware or shareware for on their own time. However, a business doesn't have preference.
The Fortune 500 company I work for has OS/390s, RS/6000s, E10Ks, and Windows* servers. When a new application comes in they don't say, "I like Windows best so it is going there," if that application is only developed on Solaris that is where it goes.
For heaven's sake a business is for the bottom line, if every application ran on Windows and processing time didn't matter, then nobody would buy an RS/6000 because of the cost; but processing time can cost money so if an application runs on Windows and AIX and you need to have thousands of users running concurrently and processing thousands of units of work then AIX becomes the preference. Say an hour of work costs $1,000 and Windows will process the workload in 15 minutes but AIX processes the same workload in 2 minutes. AIX is lower cost so that is the targeted platform.
If an application runs on AIX or Solaris and a company runs both then which is chosen? It may come down to workload and available servers in the environment. Which environment has the most capacity available may be the deciding factor.
As far as Linux in the enterprise, I don't seriously think that Linux will be a contender for the role of challenger to AIX/Solaris/HP-UX in the short term if ever. AIX has many features available that Linux does not have, plus one individual does not control the kernel. Also, there may be applications and IBM is supporting Linux and doing Linux development, however, as one application on Linux that is being brought into our envrionment let me tell you of that problem. iPlanet is our web server, however, it is running on AIX and Sun took over iPlanet when the Sun-Netscape alliance dissolved a while ago; Sun doesn't develop for AIX so the company is moving the web server to IBM HTTP Server on Linux. Well the company also uses Network Dispatcher, no problem, it runs on Linux, however, Network Dispatcher ONLY runs on a specific kernel level, which is different than the level that HTTP Server needs to run. Therefore, there is an additional serer needed to run ND which wasn't required on AIX. So no, I don't see Linux taking over the Unix world and forcing out the major Unix flavors. It is merely being adopted by IBM and Sun not so they will eventually start making it their main OS, but to give them more options and businesses more alternatives. If you can run Linux on an IBM mainframe then maybe IBM can sell hardware to a business using the lower cost solution. Though I don't see the lower cost, and besides you still need someone who knows VM. So what do you gain?
Anyway, that will probably start a big goings-on because people really have a strong opinion on OSs.