Actually, it depends on what you break. If you break the motherboard, past the point of recovery, no one will be able to tell that you rooted the phone. If you brick it (it bootloops or acts funny), you can just flash the stock ROM (or some other ROM). If you break the screen, unrooting it would be difficult, unless you had USB debugging enabled, which is a bad idea when you're out and about, because anyone who "found" your phone could unlock it (or, more accurately, remove any password or pattern you have it locked with). I use Tasker to turn USB debugging on when the phone sees certain wifi SSIDs, and off at other times, so when I'm where one of those SSIDs is (no, not at home - the "finder" could find my home address online, then park on my block and unlock my phone), I have adb access to the phone to unlock it or whatever else I need to do (like copy pictures I haven't backed up, if the screen breaks, and unroot before bringing the phone back for replacement or repair).
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