When I started at NI in 2003, David in I weren't out in general, and so I was naturally closeted at work. This was isolating - people asked if I was single or not, and so I said I was single. And then when people asked what was I had been up to last weekend, I would have to leave out things or outright lie.
It probably doesn't sound like a big deal, but it was very uncomfortable at the time, and made me feel dishonest because, well, I was being dishonest!
Even after we did come out to friends and "the world", I felt like it was weird to come out to coworkers, like I'd be making too big a deal out of it. (and I was still fairly uncomfortable with it myself) So I continued to be closeted at work until I left the company in 2006.
Of course, the reason I left the company was that David got a job in Maryland. I wanted to be clear to my coworkers that that's why I was leaving, not because I was unhappy at NI or anything. So I ended up coming out at a group meeting and saying I was leaving. It was fairly terrifying at the time, although everyone was supportive.
Since then, I've been out at all of my jobs and everything's been fine. I even have a picture of David on my desk :-)