Found this article on the web:
http://www.abarnett.demon.co.uk/atheism/rock.html Long made short is an assertion that the question "Can god create a rock he can't lift?" is logically invalid. This is because the question is essentially equivalent to the question "Can God find the limits of his unlimited abilities?" I agree with the writer up to this point, but then I think he makes a digression by asserting that is equivalent to "Can God cauliflower?"
I'd argue that indeed "Can God find the limits of his unlimited abilities?" is an interesting question though, explicitly with regards to the definition of omnipotence. This is because while the question "Can god create a rock he can't lift?" might rely on a logical impossibility (finding the finite limit of an infinite power), doesn't that mean that infinite power could never exist because there could be no logical resolution if say, god tried to arm wrestle himself. Obviously, infinite power can't exist, because if it did, the possessor of such power could create a logical paradox (either failing to create or lift the rock in question).