If you go under the Updates tab, does it say Always Keep this game up to date?
I've just fired up my OSX steam copy to make sure all is well, and it doesn't seem that there is some central problem (I think someone else would have mentioned it, too, if there was). It sounds like for whatever reason that Steam isn't updating your game.
One thing that might be the problem is that between 5.0 and 6.0 the game moved from the internal Steam2 to the Steam3 platform. Almost all steam games have made that transition or will do so at some point if they are ever updated again, and that's been a process since last summer or so across the steam store. Normally when this happens with a game, steam will pop up and tell you that it has to convert the game files from the old format to the new. I'm betting there's a good chance that's not happening for you here for whatever reason.
Probably one solution would be to uninstall the game and reinstall it, if making sure that automatic updates are on doesn't work. You shouldn't have this problem again with the game, as it looks like the steam3 platform is going to be around for a while (it's soooo much better than steam2).
Hope that helps!