QUOTE(river @ Jun 2 2009, 12:33 PM)

+/- 20 cents isn't a sharp or flat; one semitone is 100 cents, hence the name. if you consider that an equal tempered tuner will be wrong anyway (for an instrument like the fiddle, which is tuned in perfect fifths), +/- 5 cents is accurate enough to work with.
if you want to be really accurate, you need a strobe tuner. these are very flexible (you can tune in equal temperement, or just temperement, or perfect fifths, or anything else you like), and they're very accurate, but they're also very expensive. more likely to be found in a recording studio than someone's house ;-)
Interestingly, Peterson, who make very fine and expensive strobe tuners and a software version to run on a mac, now produce one as an iPhone app. For about a fiver!!!!
I'm just waiting for any announcements from the mac developers' conference next week before stumping up for a phone