Under the new policy, no reference to a trademark can be made (except for ads placed by the trademark owners themselves, obviously), and consequently no bids can be make for keywords containing such trademarks. Exceptions refer to usage of trademarks in non-competitive ads, such as those made by re-sellers, or in informative (and still non-competitive) ads.
The previous policy allowed references to competitors' trademarks and comparisons as long as they were "objective and informative" - the formulation makes me cringe, as it is clearly troublesome trying to evaluate ones objectivity when speaking of business competitors. As for the "informative" side of it the probability for someone to actually PAY for an ad to be informative of someone elses product/service, thats as believable just like the existence of Santa Claus.
It is really good to see Yahoo making a big step forward in regulating abuse and misuse of trademarks, even though that probably translates in some revenue loss for them, by having certain advertisers migrate to a "friendlier" place such as Google. According to Google's policies, "[...] advertisers may select trademarked terms as keywords or use them in the content of the ad. As a provider of space for advertisements, Google is not in a position to arbitrate trademark disputes between advertisers and trademark owners. As stated in our Terms and Conditions, advertisers are responsible for the keywords and ad text that they choose to use. Accordingly, Google encourages trademark owners to resolve their disputes directly with the advertiser, particularly because the advertiser may have similar ads on other sites. However, as a courtesy to trademark owners, Google is willing to perform a limited investigation of reasonable complaints."
The question is why would Yahoo change its policies and apparently offer Google an even bigger bite from the SEM cake? It is way too early to say now, and their official explanation is not entirely believable: Yahoo states to have had the users' best interest in mind, by providing them with a better experience when searching terms that contain trademarks. Though this would make a laudable initiative with a good PR potential, experts know the search market is driven by large publishers and advertisers and not by the little surfing guys. Numerous speculations can be made: threats of large legal actions from trademark owners, pressure from certain groups of interests are among the most vehiculated ones.
However, a more plausible one is that Yahoo makes preparations for a much larger scale movement destined to influence the market in a manner we cannot anticipate just yet.
Until further industry news, there's one thing to rejoice: from now on, no "better than Botox" ads on Yahoo and their partner sites!
* - the products and services covered by the new policy are: Sponsored Search, Local Advertising, Search Submit, Product Submit, Travel Submit and Directory Submit.