I'm no law student, so it's quite possible my understanding of US trademmark laws are not entirely correct, but the way I think I understand things, is: Typically, it is fine to share names with another company/product as long as the products/companies are not similar in nature. The bigger the company, however, the more they can say, "don't use our name anywhere". Which is why you will find 1 and probably only 1 microsoft. But also, as example, I work for a copany called Heartland, a medical transcriptionist company, we're on the 6th floor of the building, and on the second floor of the bulding is another company by the name Heartland, but they deal with taxes.
It boils down to how much confusion the judge preciding over the case (as you would have to take it to court to have the other company stop using your name and provide prof you were using it first.) thinks there will be if two companies/products share names.
Theres more to it than that, but I think thats the basics of it anyways.
It really gets fun when one company holds trademark to a name, but then another company uses it anyways. Case in point. Cisco owns the name iPhone. Didn't stop Apple from calling thier new gaget by that name. I think that battle is still in the courts though. *shurgs*
