replication breaks after upgrading master
The mysql_upgrade program tries to realign the mysql.user table with latest grants for that MySQL Major Release. Since you only did a minor upgrade (5.1.30 -> 5.1.47) the mysql_upgrade was needless. The grant tables for the Slave have no bearing on Replication since the Slave has to authenticate back at the Master.

Fixing MySQL Replication is just a case of repointing replication from the last know executable position and starting from there. Here is how you do this:

Run SHOW SLAVE STATUS\G and choose the following two fields

mysql&gt; show slave status\G
*************************** 1. row ***************************
             Slave_IO_State: Waiting for master to send event
                Master_Host: 10.48.20.253
                Master_User: replicant
                Master_Port: 3306
              Connect_Retry: 60
            Master_Log_File: mysql-bin.000504
        Read_Master_Log_Pos: 326579313
             Relay_Log_File: relay-bin.081210
              Relay_Log_Pos: 326579450
      Relay_Master_Log_File: mysql-bin.000504 &lt;--- Choose This
           Slave_IO_Running: Yes
          Slave_SQL_Running: Yes
            Replicate_Do_DB:
        Replicate_Ignore_DB:
         Replicate_Do_Table:
     Replicate_Ignore_Table:
    Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
                 Last_Errno: 0
                 Last_Error:
               Skip_Counter: 0
        Exec_Master_Log_Pos: 326579313 &lt;--- Choose That
            Relay_Log_Space: 326579450
            Until_Condition: None
             Until_Log_File:
              Until_Log_Pos: 0
         Master_SSL_Allowed: No
         Master_SSL_CA_File:
         Master_SSL_CA_Path:
            Master_SSL_Cert:
          Master_SSL_Cipher:
             Master_SSL_Key:
      Seconds_Behind_Master: 0
1 row in set (0.00 sec)


From the above display, your would choose mysql-bin.000504 and 326579450.

Simply do the following on the Slave

STOP SLAVE;
CHANGE MASTER TO master_log_file='mysql-bin.000504',master_log_pos=326579450;
START SLAVE;


Give it a Try !!!