I was noticing a couple weeks ago that the advent of Vista (and Apple's wonderful ads poking fun at it) make a very interesting topic of discussion.
Namely, when is it actually useful to say "Are you sure you want to...?"
I believe the answer to this is much closer to "never" than "whenever somebody gets lazy." I mean this in two ways.
First, I think undo (and clear, relevant undo) is almost always the right alternative to a confirmation. Installing a program that looks dangerous? Put it in a virtual machine, don't ask. (Instead of picking a solid technical solution, Microsoft's actually outlawed running Vista in a VM at all.)
Gmail wins a gold star here, since after you delete a message, you get a yellow-but-unobtrusive notification at the top of the screen offering to undo your action. You don't even have to know an obscure keyboard combination like Ctrl-Z to undo. I want to see more of this and fewer popup "dialog" boxes.
But why are confirmations bad? Well for two reasons: first, they interrupt flow and interaction. For apps that are good enough to encourage these things, dialog boxes are an awful interruption. Secondly? Because nobody reads them, and so the default behavior gets chosen most of the time. And a third reason as well: because they often involve limitations in the software that are due to laziness. ("This is broken. Are you sure you want to do it?")
We've let this disease creep into Picasa a bit, and I intend to fix it wherever we can.