This is a complicated question.  The history of the United States and the vague relationship with Taiwan.  The topic you have raised here is quite interesting and deeply complex.  First, it is of great importance to note China's surge as a global superpower both economically and militarily.  However, with this in mind, it is equally important to note that as Americans we take our advanced military technology as the global norm.  This is not true.  Currently, China does not have a functional naval aircraft carrier.  Such an invention is considered common place to Americans.  With this in mind, one must look at any speculation involving the United States and another sovereign country as the United States being at an inherent advantage from a technological stand point.

 Having set a rather hodge podge group of ideas I can now address your topic at hand.  Will the U.S. defend Taiwan?  The first obstacle we must tackle in this debate is the likelihood of such defense being necessary.  It is my belief that such a likelihood does exist. China considers Taiwan to be essentially theirs.  Taiwan is an enormous point of emphasis for the Chinese government.  We have reason to believe that a confrontation between Taiwan and China is imminent. 

Here is the policy as I understand it.  The U.S. has made it clear that it will defend Taiwan.  To what extent does this defense go?  Well, it is reasonable that the U.S. will send part of its Pacific Naval Fleet to aid.  The Chinese have already recognized this threat and have made it clear that such action by the U.S. would result in China bombing the Naval ship and sinking it.

As a patriot myself, I have real troubles even debating whether or not the U.S. would retaliate.  Furthermore, China is not ignorant.  China understands the gravity of such an offensive strike on American citizens (military personnel).  Therefore, I come back to my point of China not having a functional aircraft carrier (though it is of importance to note that China is currently working on such technology).  It is of equal importance to note that China would be the offensive country in such an operation.  As the offensive country China would be in great need of its Aerial support for such a conflict.  Without aircraft support China would be greatly disadvantaged. 

 In summary, yes the United States has made it clear that it will support Taiwan.  While it may not be with a full military alliance it does mean the U.S. will send SOME support in ways of our Pacific Naval Fleet.  Since China has made it clear that such support will not be tolerated and will be treated as though the U.S. is giving full support (as we did in Vietnam).  Thus, China will sink the ship.  The U.S. will in turn retaliate and essentially become fully supportive of Taiwan.  Men sharing a common enemey (Taiwan and the U.S.).  However, having this conflict happen in the near future is highly unlikely as China will not attempt to overtake Taiwan until their technology of the aircraft carrier has been completed.  But yes the U.S., from my view, will in fact end up fully supporting Taiwan.