I am afraid that this is not yet possible. <selector>valid only for use in state lists from Drawable and Color resources; it does not work for strings yet.
As for your mention of setting android:inputTypeto make the tooltip disappear, this is actually a known Android bug that will eventually be fixed in later versions, so I would not recommend creating code around this function, as it will break when they will fix this:
http://code.google.com/p/android/issues/detail?id=13895
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