It seems to me that there is a pretty obvious mistake in Descartes' method; it is not so much that he followed a process of radical doubt but that he didn't follow it consistently enough. Descartes made indubitability the starting point of thought. He made the perfectly valid observation that in every "Ego" there is a necessary "Ego sum;" this is not so much a logical deduction as a tautology (thus Descartes supposedly does not need to appeal to logic, which he has already doubted--but more on this later). But by elevating this "necessity" to a foundation of knowledge, Descartes commits an equivocation. Because while it necessarily follows from the "Ego" that "Ego sum", the "Ego" itself has no necessity. There is, in Descartes, a forgetfulness of the contingency of the self. An incidental "Ego" is confounded with a necessary "Ego," simply because from the "Ego" itself, certain necessary deductions are possible.
There may also be a wrench on the Cartesian gears buried in the fact that Descartes' "radical doubt" has structure; an odd feature for a method striving for univocity. He doesn't simply doubt everything at once; there is an order to doubt, and for some reason unexplained, the self is the last, i.e., the ultimate step in that order. Hierarchy is not exactly modernity's favorite heuristic scheme... yet one of the most profound foundations of modern thought seems built on a basically hierarchical conception of being and thought, with the intellectual soul standing on top as it ever was.
At the root of the inflation of "Ego Sum" is something common enough. Sometimes a method or an instrument of investigation obscures something but is mistakenly believed to reveal something. For example, a photo of the alleged Loch Ness Monster, with a dark something-or-other popping out of the water, is in all likelihood a smudge; an obscurity mistaken for a revelation. The tautology of "Ego" and "Ego sum" has pretensions of eternity and thus indubitable necessity, because allegedly, time itself has been "doubted". If only Kant's discovery, that time and space are not so easily discarded from thought, was retroactively applied to Descartes' discovery, the essentially time-rootedness of the "Sum" would have been discovered, and the "Ego" once again found to be contingent and hardly worthy as a foundation of knowledge.
The truth beneath the "Ego sum" is that even its status as a tautology has not liberated it from the structures of reason and logic as Descartes as hoped. If he had been consistent in his method, Descartes would have stopped writing after establishing the "Cartesian demon" and would have gone outside and started hitting people and screaming bloody murder. That is because it is impossible to isolate even something as simple as "Ego Sum" from the machinations of a deceiving demon or the vast perceptible, time-rooted world in which we find ourselves.
In short, it might be true that there is a tautology of "Ego" and "Ego Sum". But there is equally as much a tautology between "Ego Sum" and "Ego Sum Nunc." This wrenches the pretentious statement away from eternity and necessity and directs the radically doubting method away from the self and toward that upon which the self is contingent.