The Fugue

Google has some answers, some of which are bogus and some of which don't apply to all switches. It's a lot easier than Google would make it seem, though, if you have one switch with a known password and a management module. Just stack up the other switches, log into the management one, and do system initialize. That will initialize the whole stack (reset to factory defaults, including password but not including IP addresses). If initializing isn't your cup of tea, I system password will reset the passwords on the whole stack as well. (Hint: verify the stack is what you think it is with system inventory)