The best thing you can do is acknowledge it. That takes a lot of the nervousness away, because you're no longer trying to sidestep around the issue, and it makes the interviewer more comfortable as well. Usually on phone interviews I tell them, "I have a slight stutter, so thank you for being patient with me." Don't apologize, that will set the wrong tone. By thanking them, you'll put the person at ease, and you'll feel more at ease too.