Google informally notified Adsense Publishers that the number of competitive ad filters that could be put in place per account has now doubled to a maximum of 500.
Per Adsense Advisor in a Webmaster World thread, Google chose not to announce it on the main blog because "we don't want to overload our system with everyone filling their filters to capacity at once."
Competitive ad filtering works (in theory) in this way. Publishers filter out any URL that they believe is not relevant to their audience - a buggy whip ad on a auto-classified site, for example. Removing the ads that users don't click on results in a higher click-through rate (CTR), because the ads are more relevant, which in turn leads to an increase in earnings. The clicks might pay less, but since there are more of them, everything works out - happy advertisers and happy publishers.