To answer answerzombie's question: No, Groupon is not sitting on a legal timebomb. One of the reasons you would acquire a trademark is to establish a superior

To answer answerzombie's question: No, Groupon is not sitting on a legal timebomb. One of the reasons you would acquire a trademark is to establish a superior claim to a domain. A domain, by itself, is not a trademark and does not establish a trademark. Trademarks can be established in two ways: registration or actual use. Trademarks must remain in use to remain valid. Where you have competing claims to a mark, first in time takes priority. In this case, while the original holder of groupon.com may have been intending to use the domain, he apparently never got around to actually doing so before Groupon filed for a trademark. At that point, Groupon's trademark would have been "first in time". Plus, apparently the dude signed over whatever rights he had.