Clearly, the costs involved in running Solaris and RHEL platforms are not well understood, and generally favors GNU/Linux environments. This is (most of the time) untrue, since this tend to be based on personal user experience, which is in fact far different from running lots of systems in high demand production data centers.
Here some interesting readings on these subjects--costs and features analysis--from:
YMMV for sure, but I personally think that Solaris costs are overestimated, and its features are mostly unknown, or at least underused... but this is a very large and hot topic nowadays, I know.
Update #1 (2008-11-14): Go to read interesting comment update from Jim Laurent.
Update #2 (2008-12-02): Go to read Joerg Moellenkamp's entry about similar points.
Update #3 (2008-12-03): Go to read this article appearing in Computerworld.com.