Acts 28:25 And when they agreed not among themselves, they departed, after that Paul had spoken one word, Well spake the Holy Ghost by Esaias the prophet unto our fathers,
28:26 Saying, Go unto this people, and say, Hearing ye shall hear, and shall not understand; and seeing ye shall see, and not perceive:
28:27 For the heart of this people is waxed gross, and their ears are dull of hearing, and their eyes have they closed; lest they should see with their eyes, and hear with their ears, and understand with their heart, and should be converted, and I should heal them.
28:28 Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.
28:29 And when he had said these words, the Jews departed, and had great reasoning among themselves.
I think Paul sums it up perfectly on the issue of Isaiah 6:10. I can't find any universal rule, in its context and for its fulfillment and usage its pretty much settled. So I still find no reason to think that hardening is a purposeful action God does universally to prevent people from being saved, although they are not regenerated or elected anyway; this still does not work to prove the Calvinist doctrine of "passing over" as a supporting text.
Darren