It really depends on the company. Some companies (i.e. Google, Myspace) do not like people registering domains with the word 'google' in them regardless of the reason they were registered. Other companies -- especially video game companies, for example, never (at least to my knowledge) go after people creating fansites/forums for their games.
If they send you a C&D, I would do just that. Myspace is one of the companies that doesn't care much for individuals registering names containing the word 'myspace'. I'd advise against acquiring others and I wouldn't put much work into the site -- chances are pretty good you'll get a C&D.