Sun's OpenDS project -- https://opends.dev.java.net/ -- looks to be a very promising LDAP implementation. I haven't gotten into it much, but as I installed it this morning, I was pleasantly surprised.
The install was the easiest of FDS or OpenLDAP. A nice gui steps you through the initial install. Replication setup was simple, as the gui prompts you to identify another server already participating in the replication. OpenDS, by default, supports multi-master replication. I believe this is, in fact, the only replication it supports. I think it would be useful to have the ability to force read-only replicated servers, but I didn't see if this was possible.
I easily set up 3 servers on my machine ( a Dual-core Opteron 185 with 2GB of memory running Fedora 8 64bit ). Using OpenDS, I generated the example ldif of 10k users, and loaded it up. Replication started immediately. OpenDS provides a nice, simple gui for simple monitoring, so it was easy to see the updates going to the other 2 servers. participating in the replicated cluster.
The ldif additions were slow--it took several minutes to load the 10k users. My machine load went up past 7, and with running several servers, my computer was having to swap memory quite a bit. During the load, I shut down one server, brought it up for a minute or two, then down and up again. I wanted to see how this server would handle the synchronization when it was not up.
When the load finished, the replicated server that stayed up the entire time, had the same number of entries as the server I loaded the ldif into--10,003. The server I shut down, however, was about 50+ entries short with some errors in the replication log:
[24/Feb/2008:09:22:26 -0700] category=SYNC severity=MILD_ERROR msgID=14876739 msg=Could not replay operation AddOperation(connID=-1, opID=47, dn=uid=user.1333,ou=People,dc=example,dc=com) with ChangeNumber 000001184c39fe467f4300000537 error Canceled
Apart from that, OpenDS is off to an excellent start--especially for it's age. It's by far the easiest server to get up and running. I'll be watching as it matures to see how it performs and stabilizes.