@freemo On what conditions? What kind of keyboard?
@mzedp I need to dig up the studies to find the limitations.. but as I recall as long as the typing can be heard with reasonable enough quality it should be able to do it. It goes off the unique sound each key makes.
@mzedp Ohhh touch schreen keyboards... I dunno, it would be harder to do with sound, maybe not possible.. but a simple RF based solution that reads the signal passing through the phone could probably do it.
Either way im sure you could get that information though the approach may be a bit different is all.