The Yahoo Search Marketing blog has announced that those utilizing the Sponsored Search account functionality can now block 500 domains instead of the standard 250.
While this is helpful--after all, the number of blocked domains has now doubled--there's a concern that Yahoo is still sending "garbage traffic" to YSM users (from Yahoo search partners). One mentions that 500 blocked domains is not nearly enough. One mentions that he wants to block all non Yahoo domains.
On the other hand, one forum member says that he hopes that Google sees this as a challenge and also ups the number of domains that can be blocked.