hey there, i just bought CS:S yesterday for my imac, and i was having a really hard time playing it. it worked yesterday (connected to the internet via airport extreme) and everything was good. but it would go in and out every few hours and not let me connect. today i tried to play, and it would not connect at all. after a bit of research and reading, i found this is a common problem. i found this site and it worked.
Best Mac Games and Tips and Tricks Blog! steam cannot connect to the server problem

figured that it might save some of you some time if you come across this same issue. cheers and good luck.