Yesterday, Gibson Guitars released its self-tuning Robot Guitar. It'll add about $800 to the price of a new guitar (it's available only on mid-range guitars with street prices over $2,000), and will compete against a self-tuning guitar add-on from TransPerfomance, which runs about $3,000, including installation.
The technology behind self-tuning guitars is far from trivial--a CPU in the guitar neck must continually monitor string tension and adjust the pegs accordingly--and I'm sure the demos are fun to watch, but I think the vast majority of guitarists will get a lot more value out of a standard $100 tuner pedal. The main problem is intonation. When you buy a new guitar, somebody has set it up so that the strings are in tune with themselves--that is, when a string is in tune in the open position (no finger on the fretboard), it'll be in tune at every important interval above open, including third, fifth, and octave. But perfect intonation is rare, even on an excellent guitar that's been set up by a master. And more to the point, intonation changes over time--different strings lose tension at different rates (this can happen even with temperature changes), the bridge and nuts get looser, and in extreme cases (like if you frequently lean your guitar against walls instead of always putting it a stand or away in your case), the neck might bow.
So, while self-tuning guitars may be perfectly in tune on open strings, they won't be perfectly in tune all the way up the fretboard unless the guitar has perfect intonation.
With a chromatic tuner, you can aim for the best possible tuning throughout the entire guitar--maybe you let the open tuning be slightly flat on each string so that the third and octave are in. There are other tuning issues where manual control is nice--for instance, you might hear that another instrument has gone slightly out of tune in the middle of a song. If you're a small enough ensemble, and you trust your own ears, you might adjust your own tuning slightly to compensate, then indicate to the other person that it's time to tune up after the song's over. Or, like many lead guitarists (notably Eric Clapton--scroll down to "Why Does Love Have To Be So Sad?"), you might prefer slightly flat tuning so you stand out from the rest of the band, or so you can use thick strings (for a fatter tone) and still bend them (for blues licks).
Of course, the number one rule for tuning on stage is: always use the bypass! Nobody wants to hear you tune your guitar.