BMW is sueing Nissan in Canada over the use of the letter "M" in it's new ad campaign for the M35 and M45. BMW reports that it has invested approximately $260 million in branding the letter. In the commercials in Canada they've had the slogan "the M is coming" which BMW is alleging trademark infringement. As of yet BMW hasn't been able to prove any harm and will need to in order to win this case. In similar history, Audi sued Nissan over the letter "Q" and Acura sued Ford over the use of the Lincoln MKX. Acura believed "MKX" was too simliar to the name of their SUV the "MDX".
I guess this is great business for corporate lawyers.