Receptional_Andy - 4:23 pm on Jan 31, 2011 (gmt 0)
I assume that you're referring to trademarks here.
There are no hard and fast rules, as it depends on the legal situation in your particualr country - and that may not be well-defined, if there have not been cases that have gone to court and resolved conclusively.
That said, in many countries, it is generally accepted that triggering an ad using someone else's brand name (i.e. bidding on those words) is likely to be acceptable. However, displaying a trademark (using it in your own text) is much more likely to get you into trouble.
If you want specific legal advice, though, you need a lawyer.