I think it would cost a lot of money to invalidate a trademark, especially if the company that owns it has deep pockets and even a UDRP complaint can cost in the range of $5k from what I've read. I think that's why so many mediocre .coms sell in the $2-4k range. It's cheaper to pay the domain/trademark squatters for the guarantee of getting what you want than it is to (maybe) prove their registration was in bad faith.