An upcoming build of the CyanogenMod operating system includes a simple but brilliant security feature called "Scramble PIN Layout." It's exactly what it sounds like: a way to scramble the keypad when you unlock your phone so that people can't peek at your keystrokes and learn your PIN.
Why don't all phones have this feature? CyanogenMod is a modified version of Android that takes just a bit of technical know-how to install. But the feature is so obvious and clever, there's no reason it shouldn't come with an Android or iOS device.
If you're paranoid about somebody watching where your finger goes when you type in your PIN, this will indeed throw a wrench in their surveillance techniques. It will also throw a wrench in your ability to quickly unlock your phone, especially when intoxicated. Security doesn't come without sacrifice, though.