it would only be weird if you broke up with one of them. weren't things on and off w/the butthead for a while? i could see that it might get sticky, but then again it might not and it could be a great thing.
Nothing really wrong with it, but I personally wouldn't want it happening. My worries would be
(1) that they would become too buddy buddy and would start hanging out alone together after work. I don't like my friends to hang out with my boyfriend without me. That's just how I am.
(2) that when we all hang out together, they are going to be laughing and joking about their office the whole time and make me feel left out. I don't want to feel left out when hanging out with my friend and my boyfriend.
(3) if my boyfriend and I broke up, she would still work for him and I would both want to know and not want to know what he was up to at the same time. I've been in this position before. I simply can't resist asking the friend what is going on and then I feel bad when she tells me about a new girl he's seeing.
(4) they will continue being friends after the break-up if you were to break up.
(5) your friend or boyfriend will put you in the middle of things if there was some sort of conflict in their working relationship.
I think some of my concerns might come off as insecurities and you may not have the same feelings, but I just think there are millions of lawyers at millions of law firms that your friend could work for and why risk things being weird?
ooh--andrea julia's points are good--i hadn't thought about the awkwardness of b-f and friend joking about office politics while all out to dinner together. i would hate that.
but on the flip side, it's not weird at all between me and a good g-f of mine (a paralegal) even now that things ended hideously badly with my ex (a lawyer, her boss). she worked for him before i knew her, though, but it's pretty much the same thing. when the breakup was newer, we had an agreement that we didn't discuss the ex. now that i am not invested emotionally anymore and i am not on speaking terms w/the ex, we laugh about what a dork he is and it's fine. but it never really affected my friendship with her.