Online quizzing and claims of 'instant feedback'

I was reading another course article for my Computers and Education class this week entitled Emerging Technologies by a professor at Viginia Commonwealth University called Bob Godwin-Jones. At one point he talks about a U of Victoria based online quiz editor called Hot Potatoes. It allows users to construct short quizzes in the form of multiple-choice, fill-in-the-gap and even short answers and in class I actually went in and explored this tool a little bit. The idea is really neat, simple and completely user-friendly (as setting up a quiz doesn't require knowledge of HTML code, JavaScript, etc.).

With that said, I still had one concern. In his article, Godwin-Jones explains how one of the most valuable features of these types of online testing systems is that the student gets instant feedback after answers are entered in. This was somewhat misleading to me. Aren't short answer questions often times subjective? If this is the case, then how could there possibly be programmed instant feedback? I decided that the short answer questions are limited to a certain type of question, like, for instance:

"What two provinces border Ontario?"

This type of question would work because the program would be able to detect if the correct answers were typed into the text box, as there are only two. A question that would not work as well and would be impossible (in my eyes) to program appropriate feedback is something like:

"Using termonology and concepts you learned in class, name your favourite 17th Century painting, its artist, and why you like it."

There are an infinite number of possible answers to this question, so how could there be instant feedback? The teacher needs time to read this answer and assess it appropriately; this is where we can visibly see the limits that exist in terms of computer capabilities. My question is this; will computers ever be able to grade an entire essay, puncuation, sentence structure, content and all?