Today, the US Supreme Court held that a defendant waives the right to receive exculpatory evidence by pleading guilty. This ruling should lay to rest hundreds, if not thousands, of frivilous postconviction writs that accuse defense attorneys and prosecutors of keeping evidence from the defendant.

I think you are overreaching in your description. The Supremes said that the defendant could sign a waiver to receive impeachment evidence. That does not mean that the duty to disclose exculpatory evidence is necessarily waived. For instance, the defendant would still have a right to be told that the amount of the drugs for which he stood charged was not of a sufficient amount to warrant the offense alleged. That would certainly be exculpatory, and not subject to the agreement reached between prosecutors and the defendant in the new Supreme Court case, U.S. v. Ruiz. http://a257.g.akamaitech.net/7/257/2422/24jun20021130/www.supremecourtus.gov/opinions/01pdf/01-595.pdf

Oh, I don't know. It seems to me that would better be described as a breakdown in representation. I think the SC is clear that the due process right to be notified of exculpatory evidence arises from the need for a fair trial. When the defendant gives up his right to a trial, much less a fair one, the due process requirements disappear.