Google has announced a change in their policy on use of trademarks in Click Ads. It’s a liberalization in that there will be more situations where you CAN use someone else’s trademark. But there are serious limitations:

Those that won’t be able to use brand names include sites that sell counterfeit goods, retailers that primarily sell a competitor’s products, advertisers that criticize the trademarked brand and those that do not refer to a landing page with a purchase option.

Nevertheless, there are situations where this will be very useful, as to promote a tradmarked brand that you are selling online.
The change goes into effect June 15th.