"Google’s Trademark Policy"

Google Adwords: FAQ:
Google's trademark policy is fairly simple. The trademark holder governs how the trademark is used as a search term. There are two options: a blacklist of advertisers who may not use your trademark and a whitelist of advertisers who may. For dealing with problems, read Google's full trademark complaint procedure.

Overture's policy is a bit more permissive, allowing advertisers to use a trademarked term as a search term for "comparative advertising, sale of a product bearing the trademark, or commentary about the trademark owner or its product." The policy acknowledges some "fair use" of trademarks results in consumer confusion.

If you have a trademark issue, you may get different resolutions from different engines. Read each engine's policy, and start discussions armed with the appropriate facts and supporting documentation.