Yo,
You can't replicate from lower versions to higher versions, in general. Only the reverse is safe. So if the master is 5.1, we can assume all the slaves are probably (at least) 5.1.
Hmmm, I think that replication can be defined as (from)master(to)slave, so you can replicate from lower versions to higher versions. Not opposite, unless you're using trainwreck ;-)
Domas