I can write a greasemonkey script to mimick Gmail's "undo send" function by delaying the PM sending for 10 seconds; would that be in demand?

Hmm, maybe. Though the option to just edit it if it hasn't been viewed would be much better, imo. Delaying the sending wouldn't help if you're in communications with someone and upon further experimentation/research you realize what you sent was wrong 10 minutes later.

Besides, I'm not sure if that's even something that would be understood. For people like me who don't typically read the site news, I'd be unaware that it's supposed to delay being sent unless there's a timer. Maybe the button changes from a "Send" to "cancel? #" where # is the number until it sends? That would prevent people from accidentally sending multiple PMs with the same content. The actual implementation of the changing button wouldn't be too hard; we already have the buttons about quick replies that change between the initializing and terminating BBcode.

Even just being able to delete the PM before the person sent it would be enough, if nothing else. That way you could just delete it and resend the edited version at your convenience.

I like that, but I had an issue once on another forum because I sent out a mass email, then nobody responded, so I had assumed either nobody got it or some shenanigans had happened. It turns out, this forum had a system like the one you're suggesting, and instead of clearing my sentbox, which had a limit of 50 PMs anyways, I had deleted the sent messages.

So if we get a system that allows you to delete unread PMs or edit them, we need to make it absolutely clear to the person editing/deleting what's going on. For the delete method, a simple difference of "delete from sentbox" or "delete unread" buttons should suffice. For editing, make it so that it reads as "edit", if that's too hard to understand then fooey. And, rather than having the sentbox register if they're unread by the sender, can we have messages marked as read by the recipient? It makes no sense that a message sent by us should be marked as unread by us...

Split into two folders: Outbox and Sent