As far as I can see, Carne Ross of Independent Diplomat presents the best synthesis of the raging debate. Two summarizing quotes from his article:
"Governments around the world are this week telling each other that nothing has really changed and that if they restrict the circulation of those really sensitive telegrams and glue up all the USB slots in their computers, that this won't happen to them. But it will."..."The only way for governments to save their credibility is to end that divide and at last to do what they say, and vice versa, with the assumption that nothing they may do will remain secret for long. The implications of this shift are profound, and indeed historic." [Emphasis added]
It will be somewhat harder for diplomatic organizations to do secret bad things, although they will continue to do them. Diplomacy existed long before the internet. I'll bet the fax machine manufacturers are happy to have this niche market cornered for some time to come.
However, I think that practicing what we preach may become more common than it was before. It will be hard to tell for sure.