Canvas fingerprinting by itself won't uniquely identify users. But the idea is that you can combine various different techniques, each one giving you more bits of uniqueness, until you have enough to do so. For example, say that canvas fingerprinting gives you one of 100 possibilities, and you combine it with other techniques that give you one of 10,000 possibilities, then combined (assuming they're not correlated) you get it to a million, letting you uniquely identify people with decent reliability from a decently large visitor pool.
Good question. I'm not particularly informed on this stuff, so take this with a grain of salt, but my understanding is that mobile devices in general and iPhones in particular are much harder to fingerprint reliably. Things like time zone, clock skew, and ping times might help differentiate users, but you probably can't get it down to a single person. I imagine there's still a use for fingerprinting which helps you differentiate groups of users even if you can't narrow it down to just one.
