I don’t know how it is in the southeast, because I work at a very casual company in the SF Bay area, where being gay or straight is pretty much a non-issue. But I still never really “came out” at work, just because it has absolutely nothing to do with my job. Like you say, it makes things easier just to be out with it and get it all over with and go on with your life. But at the same time, if you make a point of it, people make a point to remember it and use it as the first thing they think about when they see you. I think piece-meal is just the better way to go all-around.
I did just come out and tell my closest friends (about four or five of them) at work, partly because I was still in my “I want everyone to know” phase, and partly just because it was easier to get it over with so I wouldn’t have to explain it if I made some comment during lunch.
Everybody else I figured would find out whenever it became an issue. My best friend here comes over to my cube every once in a while and loudly talks about his dating and asks me what’s going on with “your man” and such, so I guess he’s getting the word out for me. (For some reason, I tried for a while to be more discrete about it, avoiding gender-specific pronouns and stuff like that, but finally realized if he were so comfortable talking about it, there’s really no reason I shouldn’t be).
Other cases have been when someone asked me directly (“What are you doing tonight?” “Going out on a date.” “Oh yeah? What’s she like?” “Well, she’s a man, for starters.”)
Or when making a joke. (MY BOSS, in the breakroom: “Have a doughnut.” ME: “No thanks.” BOSS: “Fruit?” ME: “You don’t have to be rude. I’m just the way God made me.”)
And then, as will surprise no one on the SDMB, I mention it whenever the topic of same-sex marriage comse up, as it has a lot this year. Usually in the form of a sarcastic comment like, “Oh yeah, George is a swell guy. I’m going to invite him to my wedding. Provided he can make it up to Canada.” A comment like that made on the company’s not-work-related mailing list is the closest I’ve come to actually announcing it to everyone. And I didn’t think of it as such at the time; I didn’t even realize until a friend replied to me with, “Dude, did you just come out to the whole company?”
So, uh, my point is: I wouldn’t mention it at the job interview unless it’s somehow actually relevant to the job. Why should they care one bit if you’re gay? As for everyone else, once you find out who your closest friends are there, then tell them when it seems like something that’ll make it easier to talk about – for instance, if they’re the type of friends that you talk about your personal life, or if they ask you if you’re married or have a girlfriend.