Interesting. I can't believe I missed this thread. I've been too focused on my new job lately. Anyway, for my 2 cents:
I've always thought that a system that took the best of what a lot of people already do (and made it easy) would help. That is: have multiple email accounts with different uses. To formalize it, have a public address and a private address.
The public address would accept anything from anybody with the idea that you're probably not going to check it as often because a lot of junk ends up there that you just have to delete anyway. But when you want to sign up for an e-newsletter, join an online forum, etc. where an email is required and you're wondering if providing it will generate extra spam, this is the address you'd use.
The private address would only accept mail from a) pre-approved addresses and/or b) would require the sender to respond to an auto-generated message. The definition of "pre-approved" is where it gets interesting. You could exchange keys for example.
But you could extend this (with a lot more work) to include membership in groups that you sign up for on other sites. Now the accepted way of handling that is to manually add the mailing list address to the whitelist. A simpler solution (for the end user at least) would be to have it happen automatically or not use the standard mailing list model at all.
The last nice to have would be an account that wrapped both the public and private addresses, so you could login in one place and check public and private folders.
Okay. Call it a daydream, but email and spam control is one of those topics that most people have something to say about.
