Steve Krug recommends GoToMeeting in his book (Rocket Surgery Made Easy). They've got a 30 day free trial so we're going to try it out next week on some of our colleagues to see if it's worth the price. Basically what we want is the ability to capture the screen and sound and to play this live for the developers in another room, so that we can all observe together. I looked at Silverback, but I think getting users to do a usability test on an unfamiliar device (i.e. a Mac) can only lead to problems and confusion. I'll let you know how we get on.
Royal Library of Denmark