Funny you bring this up. It happened to me last night when I launched Steam. There was a Team Fortress 2 update that began downloading, but the store wouldn't load. Then I noticed that Safari had stopped loading web pages. I checked my DSL modem (front lights) and everything was fine, but after the download finished there was no more internet activity. The mac reported it had an active IP and was connected to the internet.
I finally rebooted the modem and everything went back to normal. I just chalked it up to a Qwest hiccup, and carried on with my evening.