i think therefore i am... nah, Descartes was the father of positivism (or positive philosophy) and that was the root of all evil... for details see guys like Wittgenstein, Heidegger, Foucault (and yes i have read them. not all they've written, but the important bits)
if the illusion of a Matrix like system would be so consistent and perfect as to not be distinguishable from "reallity" wouldnt that make it the equal of reality?
level of evidence that this world is an illusion equals level of evidence that says the wold is real. it's really to big of a question. maybe some hotshot philosopher/physician (thinking Hawking, anyone?) might crack this one.
I type therefore I am, I am therefore the forum exists... couldnt resist...