I think it's a bad habit for anyone to get into - it really reflects poorly on you... but most adults, at one point or another, do sweare. Most people do when they're extremely angry.
My husband and I cuss more than we should - and we have a 20 month old who's soon to be learning to talk - so we curb it in front of him. We also don't swear in front of older family members or people we don't know... only around our group of same-age friends.
I don't think it's okay for kids to swear, just because they don't understand the use of discrection like adults do. I also don't think that "bad words" have ever actually hurt anyone, and sometimes wonder who established them as bad words in the first place....
I think that if people are selective and discreet about using profanities, including filtering things on T.V. and in movies from their kids, their kids won't learn the words until their older. But they will eventually learn them, as curse words are everywhere and almost unavoidable unless all you do is sit home and watch Dora the Explorer or Blues Clues all day.
In summary, it isn't okay for kids to curse... as I said, only because they don't understand discretion and social situations enough to establish whether or not it's okay, on the same hand as those little white lies adults sometimes tell (if they run late to work, if they miss a meeting... people come up with little reasons why...) YET, we teach our kids not to lie. Why? Because they don't understand how to judge a situation well enough.
All in all, the reality of the world is that sometimes a curse word happens... sometimes a curse word is overheard... all you can do as a parent is try your hardest to filter these things away from your children until they're older.