Update - January 29 2013
As mentioned here - comments are now disabled even for registered users. Sorry. Comment via twitter - or better on your own site. Thanks!
Given the extremely large number of spam comments I'm seeing (at least 10 to 1 over real comments), comments on articles are not posted automatically unless you are a registered site user (see the "Create New Account" link on the left-hand side).
If your comment sounds relevant to the discussion on the page or it's from somebody whose name I think I recognize, I'll almost certainly approve it as is. But there's always a chance I'll mistake it for spam. If I don't have a clue what you're talking about, I will treat it as spam even if the name seems to be from a commenter I've seen before. So write clearly about subjects that you yourself understand, or you won't get posted.
I generally review the approval queue at least once a day. If your comment hasn't appeared within 24 hours, it's possible I mistook it for spam. Try posting a second time with a note about the problem - and perhaps try rephrasing your comment so that I can make sense of it, if it seems a difficult subject.
If anybody tries to post a comment I find objectionable or abusive of individuals, groups, basic scientific facts, or is in some other way insupportable I will replace the offending material with a [removed - moderator] note; repeats will be considered spam and not posted at all. Comments that appear to be simply mocking, trollish, or otherwise intended to incite thoughtless reaction without indicating any sort of effort by the commenter at understanding the discussion will be deleted from the start. Comments from registered site users will not be moderated in this way, but abuse will be considered grounds for removal of their registered status.