If we all say "it's okay" will that be a defense to a trademark infringement lawsuit? Don't think so.
If the potential trademark holder says "it's okay, but do it this way" does that sound like a defense? Much more likely to avoid trouble.
Jeff Esq.
However, based on what you've said, I suspect it would be OK to use. If the keyword is one for which the trademark owner has already contacted Google, to prevent others from advertising on it, then your ad would simply not show for the keyword in question.
Or if the trademark owner sees your ad, and has an issue with your using the trademark as a keyword, he or she can certainly contact Google to request that you (and others) not be allowed to use it.