From experience, the other guys just go dark on you. I got a threat of libel from the companies attorney in response to my initial cease and desist. I rebutted the claim with them, but never heard back. After talking to about 10 attorneys, the estimate for fighting it in court is about $80K. Another option is to file for invalidation with the USPTO, which can be done without an attorney. Here's the whole story if you are interested: http://www.geekceo.com/entry/zoto-trademark