This also depends on what you call "exact intonation". As with most modern instruments, the guitar is a Equal Temperament instrument which means that there are some compromises made to enable the instrument to play in every key without having too many notes or having to retune for every key.
Equal Temperament is what we are used to hearing, so the compromises are not a big issue in most situations. However, in it's standard form the guitar fretboard isn't a completely accurate version of Equal Temperament - the fact that each fret is shared by six notes means that there is further compromise and this leads to some very sour intervals, particularly in the first position, where there is anywhere from -1 to +4 cent deviation from the Equal tempered notes.
The modern compensated intonation for guitar - like the Buzz Feiten Tuning System and my favourite, the Earvana nuts - don't get you closer to Just intonation, but rather get you closer to a more accurate version of equal temperament, by dealing with the inharmonicity inherent in the design of the guitar fretboard. I have to be honest, once you get used to the added sweetness of chords in the first position, there is no real going back...
Probably this leads into a discussion on tempering and just temperament - something I don't understand properly.
How many really do? I recently had a great email discussion with a piano tuner/guitarist on intonation, temperament, inharmonicity and more. It gets incredibly complicated, and easily half the discussion was way over my head, but I keep on going back to read through things again, and each time a little more bludgeons it's way to my understanding. I suspect that I'll never fully understand it as I don't have the math skills required for a complete understanding.
