trademarks, when they are commonly used as if they were ordinary, generic terms, exist in a kind of never-never-land. to the lexicographer, the word has become generic and should be included in the dictionary and defined.

here's the OED2 entry for Xerox
[Invented word f. XERO(GRAPHY.]
n. A proprietary name for photocopiers (see quots. 1952, 1953); also used loosely (attrib. and absol.) to denote any photocopy.
v. [f. prec.] trans. To reproduce by xerography; to photocopy.