.info domains are currently quite cheap (under a buck individually, and only a few cents if bought in bulk), so the spammers can buy a load of random domains that aren't recognized by current spam-controls, and set them up to patrol the web for known forum vulnerabilities and to send out tons of spam emails. If they pump out a high enough volume in a short amount of time, they probably can make a quick buck from the very small percentage of receivers (including forum administrators such as yourself) who actually click on one of the millions of ad links sent out before they are banned. My guess is as the .info thing gets figured out, they'll just switch to some new tactic.