Oooh, that's a hard one. I'm very interested to hear what everyone else says though because we're expecting our 2nd in August; I don't know what it is yet - we might just have to deal with the same issue. My daughter is 3 though, so I'm not sure she's set on one gender on the other. One day she'll say she wants a sister, the next she'll say a brother...lol.
Perhaps you can ask her why she doesn't want a brother? Ask her what kind of games she wanted to play with a little sister, and point out that her little brother can play the same games. (I personally, have no issues with a boy playing with dolls - how many teenage boys have you seen with Barbies, and what are action figures but little dolls? - but I don't know what your and your husband's perspectives are about a gender-roles so I'm not informed enough speak to that).
I guess just try to figure out what it is she doesn't like about boys. Maybe her father does something (or doesn't do something) that she doesn't like, and she's afraid her little brother will be/do the same way/thing? I'm not saying he does bad things, of course...It's probably something very simple but that you as adults wouldn't think twice about.
Dunno, just thinking out loud here in the hopes that it'll spark something for you that might help...
Please keep us posted on how things go though, as I'm concerned I'll be dealing with the same or a similar issue in a few months!