Believe it or not, it's not Sod's Law that dictates a dropped smartphone will land screen down: it's actually the same physics that influences buttered toast in free-fall.
As part of a PR exercise to encourage people to buy its shatterproof phone, Motorola set physicist Robert Matthews -- best known for his authoritative paper, "Tumbling toast, Murphy's Law, and the fundamental constants" -- to work on the smartphone problem.
The answer is surprisingly simple: whether a phone lands screen-up or face-up depends on a few variables, mostly how you're holding the phone at the time of drop and the distance to the ground. Because people tend to hold their phones in a certain way (fingers below the center of gravity on the back, at roughly chest height), the numbers work out such that phones more often than not land face-down.
The good news is that the equation rests on people fumbling the phone when they drop it: if you just let it go, it's more likely to land screen-up. Though, I'm not sure that means Professor Matthews will pay you back for repairs if theory doesn't quite meet reality.