Cross this one off of your guerilla marketing to-do list. According to CNet and other sources, Yahoo will prevent advertisers from bidding on competitor keywords starting March 1st.
My understanding, mostly derived from hearing Alan Rimm-Kaufman speak on the subject, is that current law is meant to prevent deceptive or misleading advertising – like if you placed a paid ad that was titled "Bentley" but led you to the Mercedes site instead. The way Mazda and GM did it – and are still doing it when you query "pontiac" or "kermit" on google – seems perfectly legal.
Then again, I’m no lawyer, so don’t take my word for it – check with someone who’s got proper credentials. Is it just a matter of time before users start tuning out search ads?