With the release of Lion, Apple has actually be leaning heavily on calling it "OS X" instead of "Mac OS X". You can see this branding prominently on the OS X Lion page.
However, the platform (as opposed to the operating system) is generally called "Mac" both by Apple (e.g. the Mac App Store) and colloquially (e.g. Steam). I'd much rather see it synonymized to mac instead of the normative use of one version of the operating system name.
Note this has come up once before and oddly not completed correctly.