One striking (and well-documented) case of this sort of trademark
dillution is the Merriam-Webster dictionary
Noah Webster started the original Webster's dictionary. His heirs later teamed up with publisher G. & C. Merriam Company to produce the first Merriam-Webster dictionary in 1847.
Unscrupulous publishers rode on the success of Webster's dictionary and began publishing their own "Webster's" dictionaries, sometimes simply copying the original and republishing it without license. Despite legal actions on the part of G. & C. Merriam Company (later renamed to Merriam-Webster Inc.) the name fell into common use. To this day, there are still many dictionaries published as "Webster's" but have no connection to the orginal.
Read the story here: http://www.m-w.com/about/webster.htm