However I do think artificial sentience is very likely in the future. It won't happen with the traditional Von Neumann architecture that most modern computers are based on. They are, as you so rightly poiint out, really nothing more that advanced adding machines - based on absolutes of true and false, one and zero and fundamentally sequential in nature (even with parrallel processing each process is still basically sequential). However with the developement of ever more complex neural network systems, which much more closely mimic the way our own brains work, you will begin to get "chaotic" effects and a dgree of randomisation happening, which in turn has the potential to lead to "new" spontaneous, "thoughts" which in turn has the potential for self awareness. This is starting to happen now, though there is still a long long way to go; we have systems that can learn and, theoretically at least, learning is an unending process. I still think future self aware and sentient computers are very high probability.