resetting superuser password

Howdy all,
  We do not know the superuser (postgres) password to our database
cluster.  The auth configuration (pg_hba.conf) has the only auth method
set to:

Type    db    user    method    param
local   all      all      md5         sameuser

I have root access to the machine, but I don't see how to reset the
superuser password. The thing I tried was to briefly replace the above
auth configuration with

local   all      all      trust

and then reset the password (after the required SIGHUP).  The password
update apparently succeeded, but when I put the md5 auth method back in
place I still did not have access.  I conclude that "regular" and md5
passwords are not stored in the same place or in the same way, which
isn't all that surprising.   Any tricks you can send my way would be
most helpful.
Cheers,
Andrew