Clinton's "change you can xerox" line last night bombed miserably, but it's actually an interesting example of how a particular product can become so popular and wide-spread that its name starts to replace the actual noun. Xerox, of course, is actually a company that makes photocopier machines, and have had such great success that people now think of photocopying and xeroxing as synonymous.
While some people think that this is just great PR, it's actually really bad for the company in question. If a trademarked name becomes so generic, a judge can actually rule the trademark invalid, the company loses its name, and everybody can start (legally) calling their products by the same name. All in all, the bane of marketing.
Just for kicks, here're some other commonly used words that actually started out as trademarks:
coke (fer the sutheners)