Well a few select people on this board know that I have had some run-ins with Mastercard over their "priceless" advertising campaign. They tried the same bully tactics with me on my site
www.thisispriceless.com. After getting advice from some of my peers I decided to fight Mastercard on it. I didn't want to give away 100k impressions a day and tons of SE traffic to those bastards!
I hired the best copyright lawyer in Canada and threw some money at it. Long story short, Mastercard has backed off and I haven't heard from them in 6 months.
The only advice I could offer you would be to sum up how much you can afford to spend on the defence, how much you make from the site and if it's worth fighting for?
I say fuck em! Good luck!
DH