If (A) as an answer doesn't work for you then I have an even shorter version of an answer.
(B) Don't.
All other versions are replete with risk.
Let me put it this way: If you're afraid to ask - even if it's not fear, if it's any other rationalization - don't you already have the answer? I'm mean, if you are rationalizing avoiding asking that suggests that you kinda are tuned in to the situation already.
I'm not going to debate this issue. My answer is what it is: Ask and get it in writing with terms and conditions or don't do it. If you choose any other path you expose yourself to all manner of incalculable risk, including the risk that you just happen to be the person that law firm X or company X chooses to serve as the poster child for what they will now do to anyone who treads on their trademark. (Think bankruptcy for you. Loss of assets. Etc.)
Will you be smiling in the poster picture? I think not.
[edited by: Webwork at 12:02 am (utc) on Jan. 10, 2007]
Most firms are very protective of their name and are not eager to give others any kind of control over their enterprise. But, you can ask. Good luck.
A classic case is WWF. The World Wildlife Fund (WWF) granted the World Wrestling Federation (WWF) limited use of their trademark (WWF).
When the World Wrestling Federation got popular the World Wildlife Fund decided that the World Wrestling Federation was diluting their trademark. So after a nasty court battle the World Wildlife Fund prevailed and the World Wrestling Federation had to change their mark to WWE.
So, even with written permission you may also run into problems down the line. There are so many issues to consider when getting it in writing. Such as advertising rights, selling products, disclaimers...
It won't make them any money they would not have made without you. But even if it does, they'll send the lawyers.
It probably won't make you money.
If it does, the lawyers will come for it. All of it.
If it doesn't, the lawyers may be slower. But they'll come.
You know all this. Why ask?
Pleading ignorance in 2007 will not cut any ice in any court in the world.
Anyway, why be a parasite? If there's money to be made, join their affiliate scheme and be done with it.