After many problems I managed to finish an upgrade to 5.0.10 and all was working fine until I restarted.

ldap wouldn't start because slapd wasn't started.

Launching slapd manually as zimbra produced the following
zimbra@server1:~/libexec$ /opt/zimbra/openldap/libexec/slapd -d -1 -f /opt/zimbra/conf/slapd.conf -u zimbra
@(#) $OpenLDAP: slapd 2.3.43 (Aug 20 2008 12:21:57) $
daemon_init: <null>
daemon_init: listen on ldap:///
daemon_init: 1 listeners to open...
daemon: bind(7) failed errno=13 (Permission denied)
daemon: bind(7) failed errno=13 (Permission denied)
slap_open_listener: failed on ldap:///
slapd stopped.
connections_destroy: nothing to destroy.
Starting it manually as root works fine, and zimbra can be started and runs fine.

Seems to be a permission issue, but I've run zmfixperms as root and the problem is not fixed.