This is a modified version of my answermy answer on I think system specifications should show as signature?I think system specifications should show as signature?.
I've seen similar systems implemented on other sites (like the Apple Discussions).
The current system is to specify the OS in the body of the question and/or in a tag. I think that way works pretty well; it gives users an easy way to include the info and gives us an easy way to organize things.
The idea of making people know and tell us their OS is good, but I don't think it would work well on Ask Different.
(Note: I'm assuming we would require this information for questions tagged with any of the OS tags.)
Detailed OS info isn't always necessary. This would force people to include info even when it isn't necessary (not all questions tagged osx need a version specified).
Dedicated OS info would frequently be redundant. We'd run the risk of causing redundancy by requiring people to specify their OS version if they have already specified it in the body of their question or in a tag.
I don't think that adding system info separately is the right path to take.
What I would like to see, though, is another 'reminder' item shown while posting a question that tells people to be sure to include all relevant hardware and software information.
Of course people wouldn't always follow it, but commenting and asking for info is easy, and people generally respond to it.