A bigger problem than being able to try over and over again is being able to bypass screen locks entirely, or being able to figure out the way to unlock the screen so it only takes one or two tries, and so on.
I've seen buggy behavior where shutting down a smartphone and restarting it might cause the screen lock to reset, so one doesn't need to unlock it at all.
I've also noticed that with many smartphones' unlocking pattern implementation, people leave streaks on the screen where their fingers have traveled the same path over and over again in a given day to unlock the smartphone -- thus allowing someone with half a brain to figure out the pattern in two or three tries (easily within the realm of failed attempts for a normal user who is actually supposed to know the unlocking pattern).
Wiping the device after 10 failed attempts to enter the unlock pattern isn't good enough, and if you're using an actual password for which you need to use the keyboard, you're probably going to get very sick of entering that password after a few weeks of use. The sad fact is that with these tiny interfaces and no better input options than we currently have, we're unlikely to have any kind of authentication mechanism for the things that will be both effective and usable with any convenience at all. The interface has to improve before security can really improve in this case.
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