If you've gotta ask the question, you're likely goofing off and avoiding work and asking the SO community to help you justify that. However, it can be legitimately used. I save no end of time researching answers to problems on boards like this. So I see absolutely no harm in answering questions as I come across them.
As long as it doesn't impede my work and I'm meeting all my deadlines for the various projects I have on the table, my code is good quality and I'm not constantly goofing off, then great - it's a tool just like MSDN but interactive.
However, if you're being paid by the hour to complete your work (such as a contractor), then morally, should an employer be obliged to pay you to sit on StackOverflow all day long? Of course not - they're paying you to do a job to the best of your ability. When you choose to give back to the community is your own decision - but your employer probably won't be as understanding as you'd like them to be.
You do have a far greater potential to get sucked into not working by a whole forum of developers wanting to bounce ideas of others than if it's just a couple of coworkers though... I dunno, I think I'd say it's 6 of one and half a dozen of the other.
Toss a coin and there's your answer.