Self awareness. I'm not talking about "emotions" or anything like that - those concepts would probably be pretty much abstract for it, even though it would understand them. I'm talking about an artificial intelligence that knows it exists and knows what it is - even though it is dedicated to performing any task the programmer designed it for.
That could be done even today, though. You could just program a computer to know that it exists.

To really
know something is to be able to expand upon it (i.e. deduce other stuff from it), so as long as that the fact that it is a computer is in its knowledge base that it uses to determine stuff, it would be quite sentient.
I think that a truly "sentient" computer can be described as one that can pass the famous Turing test for all five senses, at which point the human and the computer would have merged into one.
A true sentient AI would need hundreds, if not thousands, of paralell processors, like our own brains. The problem is, this is a much more complex system, and thus, slower. Not to mention it would be an engineering nightmare.
Actually, it would be far simpler, mathematically speaking, than the human brain is; one of the reasons the human brain is slower than the computer in certain ways is because of its more complex nature, but then it can also handle a wider range of tasks. And just as thousands of transistors are on an integrated circuit, perhaps one could also cram thousands of processors onto a little 1x1 mm square.

and the chances of man inventing AI capable of making its own decisions/emotions are how few?
Almost certain, given the full lifespan of human civilization.
