http://cyber.law.harvard.edu/metaschool/fisher/domain/tm.htm
In the legal terminology, a trademark like either 'craigslist' or "Craig's List" would be considered a descriptive mark that has acquired secondary meaning via use, such that the consuming public identifies a mark with a particular producer. It now doesn't matter if your name is Craig and you have a List. If you name a business that and people are confused because the field of use is similar to those for which Craiglist is known, Craigslist has a valid case against you.
The Wikipedia article (http://en.wikipedia.org/wiki/Trademark) also highlights that "The test is always whether a consumer of the goods or services will be confused as to the identity of the source or origin."
Another example of the primacy of the liklihood-of-confusion standard is the following guide page at the USPTO:
http://tess2.uspto.gov/webaka/html/Likelihood/Likelihood_of_...
Small changes in appearance/spelling/punctuation (like adding a space in the middle) are not enough to resolve a conflict.
