mysql replication master 5.5 slave 5.1 error on create database
Every replication event generated by the master includes the error code that resulted from execution of the query that generated the event.  This is usually "success" (0, no error).

When the slave executes a query, it expects the error it encounters to be the same error as the master encountered (again, usually "no error").  When this doesn't happen, replication stops.  Note that the error message from the master isn't preserved, just the code.  That's why you see placeholders in the error message, because the slave displays the sprintf() template for the error message.

But note, carefully, where the error is.  

Check the master server's error log.  

It looks as if the master was not correctly upgraded to MySQL 5.5; specifically, it looks like the mysql_upgrade utility was not used to upgrade the system tables to be fully compatible with MySQL 5.5.

http://dev.mysql.com/doc/refman/5.5/en/mysql-upgrade.html 

You are indeed running an unsupported configuration, with a newer master and an older slave... but in this case, it appears that your master has a problem that is actually unrelated to replication.  

Error on master: 
message (format)=
'Column count of mysql.%s is wrong. Expected %d, found %d.  Created with MySQL %d, now running %d. 
Please use mysql_upgrade to fix this error.' 
error code=1558 ; 


The master encountered an error; you may find similar errors in the master's log, although it's possible that they only occur at startup, if at all.  In any event, your application should have seen this error also... perhaps it's ignoring it. :(

The slave didn't encounter any error executing the actual query.

Error on slave: actual message='no error', error code=0.


It occurred to me that it's very important that the actions performed by mysql_upgrade can't be safely replicated, since the slave server is older.  It turns out, this was anticipated in the design, so the mysql_upgrade utility appears to be safe to run on a master, without disconnecting the slave, even if the slave is older (or was already upgraded). According to this comment in the source code of mysql_upgrade.c:

Master and slave should be upgraded separately. 
All statements executed by mysql_upgrade will not be binlogged.