CAPTCHA on mobile: what are the alternatives?

From a UX perspective, CAPTCHA's are bad, bad, bad.

But let's say we have this as a requirement. And it's for a hybrid app (compiled app, but built upon an HTML5 platform).

I'm trying to find out:

is bot traffic from an iPhone (or Android device) actually a problem? (or--my theory--is this an outdated hold-over requirement that has been cut-and-pasted into technical requirement documents since 1998?)
If so, are there better alternatives that are mobile-centric over yet-another-annoying-CAPTCHA?

Tags: UX Stack Exchange