Ok - I upgraded my development zimbra cluster starting with the active node. Everything seemed to go ok up until -

ERROR: service.FAILURE (system failure: ZimbraLdapContext) (cause: javax.naming.CommunicationException localhost:389)
ERROR: service.FAILURE (system failure: ZimbraLdapContext) (cause: javax.naming.CommunicationException localhost:389)
Setting defaults...ERROR: service.FAILURE (system failure: ZimbraLdapContext) (cause: javax.naming.CommunicationException localhost:389)
done.
Upgrading from 5.0.9_GA_2533 to 5.0.10_GA_2638
Stopping zimbra services
Verifying /opt/zimbra/conf/my.cnf
Starting mysql

And then it just hung. I had to ctrl+c eventually. I then ran enabled the cluster service and everything started on the active node with no issues. The above messages and the hanging of the 'starting mysql' is a worry and before I upgrade my production clusters, I would like to be comfortable knowing what the above means.