I was watching "Joe Millionaire" last night and there was a sequence where the girls were introduced to "Joe". The room and all the characters were black and white, but as each girl was introduced, she was the only colored object in the room. How was it done- Chroma key and something else?

I didn't watch Joe Millionaire, but the effect you described is done by keying using a travelling matte.

There used to be a good tutorial on this technique at:

http://www.wrigleyvideo.com/videotutorial/index.htm

I just tried it, and the website appears to be down. Hopefully, it's just a temporary interruption.

Also without seeing the effect, I would say either After Effects or more likely Commotion Pro might have been involved. Colored elements in a decolored scene is a common rotoscoping job, which is what Commotion does.

I've done this effect a few times before. In After Effects I made a duplicate copy of the footage and made the lower one B&W and the top colour. My footage was of a Canadian flag that slowly turns to colour for a Remembrance Day video. I used Vector Paint to keyframe a matte on the coloured layer. Took awhile but it looked great!