Survey finds that most people use the same one or two passwords everywhere
The results of the Gartner research are detailed here: http://www.itsecurityportal.com/itsecurity_news.asp?articleid=262928
While it is understandable that people generally are not fond of having to remember a ton of different passwords, the reality is that there are some excellent FREE applications/add-ons that help you maintain password disparity. Keepass allows you to have different passwords without the need to memorize them all, while Password Hasher allows you to just worry about a master key and it then computes the unique password based on the domain name of the target website.
Keepass (runs on just about any platform, not just Windows): http://keepass.info/
Password Hasher add-on (for Firefox, and who in their right min would use any other browser?): http://wijjo.com/project/?c=passhash