TC is marketing a tuner with this feature now: http://www.tcelectronic.com/polytune.asp

I guess the concept is that you can just strum the open strings and adjust only those strings that need it.

It's kind of interesting. It looks like you may be able to monitor if other strings go slightly out-of-tune as you tune an individual string as often happens with tremolo-equipped guitars or when changing tunings. I can't see dropping extra cash for it though.