That sucks, but I'm surprised a domain was available when the trademark wasn't - usually it's the other way around. What's worse than likelihood of confusion claims are dilution claims. For marks that are well known enough, there doesn't even need to be any kind relationship. Even a small plush toy manufacturer couldn't call themselves "Microsoft." This is why so many web companies go with neologisms.
Here's a pretty good primer on trademark law: http://cyber.law.harvard.edu/metaschool/fisher/domain/tm.htm