Just go to the admin page and use the Lost Password feature, assuming you set up a valid admin email address.
if you did not set up a valid email address, go the database directly and change the option for the email address manually, then use the password retrieval to get a new password to be generated and sent to that email address.
I setup 2.5.1 cleanly, then imported the db from another wordpress installation with an export command. None of the accounts that I know the passwords for work.
I found a solution to this. If you go the db interface and reset the password for your user using an MD5 hashing function. You’ll be able to login with the password. The system will automagically do it’s thing to transform the MD5 password into the WP2.5.1 munged version the first time you login, however that happens.