If you don't accept mail from proxies, zombies and trojans, residential broadband, American university dormitories and other sundry spamhavens your spam load drops to close to zero.

You reminded me of this.

Seriously though, it seems like you spend a lot of time arguing based on assumptions which haven't been proven. If you think it breaks SMTP and have an argument that has not already been answered here already, it would be interesting to hear how you think it is broken. I'm sure the owners of the 14,000 domains that are presently using spf would be greatful to find out that there's a problem with their email that they didn't know about.

The argument about Ebay and birtday cards, etc.. seems to be without merit, because its simply not true. As the publisher of the SPF records for my domains, I can easily allow any location I want to send email from my domain. I can just as easily disallow any location as well.

A day will come when people will start assigning higher and higher spam scores to email originating from locations without SPF, and the eventually some ISPs will just start bouncing that mail completely. When that point comes, it will be harder and harder for ISPs to pump out spam meanwhile crying "it's not us.. we're being joe-jobbed". That is the point where the envelope headers absolutely will become useful, and SPF will become one useful tool in spam prevention.