This is tricky.....
Though famous names are considered TMed, they are also public figures. In return for fame, they lose privacy and are dilution of thier name.
You can own a celebrity name as a fansite and there is a good possibly you can keep it if challenged. But if you use the domain for financial gain (whether actual or future) of any type, you you then cross the line of squatting.
Yes, you can own hiltonsisters.com, turn it into a fan site and they are almsot powerless. but if you make money or plan to make money from the site, you lose that protection. This includes offering the domain for sale, PPC, subscriptions, banner ads, etc...
So usage is important in this case.
Also, be careful of the usage of Hilton since it is also a hotel chain. Different ramifications could be involved.
