It can be frustrating to deal with search engines and their keyword bidding rules. Here is an overview of paid search ad trademark policies.
Allows trademarks as trigger keywords, but trademarks in the visible ad text is not allowed. So you can add trademarked keywords, but you cannot write ads using these keywords.
Yahoo and MSN:
Do not allow trademarks as trigger keywords nor in the ad’s display text. So all of you hoping to get residual traffic from ipods, nikes and porsches are out of luck.
The exceptions are if you are a reseller, an information site (but not a pure affiliate site), providing product reviews (this is how pricegrabber.com gets away with so much), or providing dictionary terms (answers.com).
So these are the search engine marketing rules you must follow to play in the pay per click arenas of the big 3 search engines.