http://www.theregister.co.uk/2010/02/02/e_banking_password_fail_survey/

The majority of online banking customers reuse their online-banking
login credentials on other websites, according to a new survey on
password insecurity.

Online security firm Trusteer reports that 73 per cent of bank
customers use their online account password to access at least one
other, less sensitive website. Even worse, around half (47 per cent)
use the same online banking username and password for other website
logins.

Read more at the link above

What the article does not very clearly detail, is the various credentials associated with other websites (such as games or forums) are frequently cracked (due to bad security deployments)  and automated programs then will go and try these same credentials (logon ID and Passwords) on the top 50 or so online bank websites. It’s even possible that lists of thousands of these cracked accounts can the sold to various underground groups and they will use them to try even other online sites.  In short –  don’t use the same credentials on your bank sites as ANY other site. 

I suggest some type of password vault to store your account names and passwords. These vaults should be kept on a thumbdrive (that you keep in your possession) that has a master password.  The U3 (http://software.u3.com/ ) removable media ship with some of these password vaults (you might have to pay for a one that supports more than about 10 passwords).