Trackback SPAM?

As part of PITCH we are working on developing a clean growth path for comments attached to a published article to grow into a published article of their own. As we have envisioned many uses of trackback for this purpose, it occured to me: why isn’t there trackback SPAM? Couldn’t a malicious person send a trackback ping (which pointed to a service or product they were trying to market) to any article in any blog supporting trackback? I can hear it now, “Blammo! Instant links to my product on 100,000 sites!”

Of course the offending party can’t hide they’re identity like they can with forged mail headers, but would this be enough to stop unscrupulous people from trying to pick up a little free advertising? Has this been discussed elsewhere? What solutions have people employed to avert such behavior? If there is not a clean solution to this problem, the death of the blogosphere as we know it could be much closer than we think.