I Just did a trademark search and yes, the word wi-fi is trademarked but so is wi-fi guys,wi-fi spots,wi-fi world......etc All to different company's so i am sure there is not a trademark issue unless i reg'd just the words wi-fi.
Well, that's interesting. $XX then. $100 tops, but only because the dash is a bit difficult to type, but as it is a part of the identifying brand, it isn't hurting it much and mabe you could actually get a couple hundred from the right party.