Well the thing about the ITC is that one of its stated responsibilities is to advise the President on international trade. This strongly implies that the purpose of the ITC is to serve the President's (and executive branch's) decisions in international trade. Knowing this, it really doesn't seem that unusual or improper for the President to intervene on the ITC's behalf.
Aside: to almost everyone else in this thread, don't focus on the Apple vs. Google aspect of this story or the specific patent claims. While perhaps a worthwhile topic (one I probably think has been discussed to death) this story is not fundamentally about that.
Is there any precedent for a President doing this?
It doesn't seem wise either from a political or from a good-governance point of view for the President to wade in and overrule decisions made by other bodies.