We have 2 MySQL server in a Master <-> Master replication, but with an Active/Passive configuration. They're both using version 5.0.75. It's time for us to upgrade MySQL for various reasons and I wanted to make sure this wouldn't cause any issue to have temporarily 2 different versions of MySQL in the replication. Also, if you have any specific process to achieve this, I'm happy to hear it!
I've done just this quite recently - going from an already mixed combo of 5.0.51a and 5.0.67 (that had been working fine for ages) to Percona's build of 5.0.91 at both ends; this also freed me from standard OS packages as Percona supports the same builds across multiple Ubuntu releases. FWIW, I asked Percona about it beforehand and they thought it wouldn't be a problem. Since then I've also done a 5.0 to 5.1 upgrade via replication too - upgrading the passive master first, switching apps to it via mmm, then upgrading the old active master. All worked fine.
That said, I thoroughly recommend you back up using xtrabackup first. mysqldump backups take forever to restore and are less reliable.