HA distributed LDAP service

This is kinda off topic, but this is the best place I can think of to
ask the question.

I'm working on deploying an LDAP Authentication service for my company.
we have 10 separate offices, and our goal is if any of the offices get
isolated from the WAN to be able to still have login service.

The simple solution to solve the network availability problem is to have
replicated login servers at each location.  But if one of the servers
dies, the site will be without login service until it's repaired.  So
now I'm trying to find a solution that has high network and system
availability. 

I've come up with this plan:

Have a master login server that serves the requests for all the
locations. At each location have a backup server that listens for the
master's heartbeat. If there is a network outage where a site is
isolated from the network I want the backup server to acquire the IP
address of the master.  

The difficulties I see are:

- If more than one office is on the same isolated network segment, to
have only 1 backup server try to be the master, and have the others
monitor the acting master.  

- When the network comes back together, to have all the acting master
relinquish the IP back to the master server.

So what do you think, is it a good plan? If so, are there tools that can
handle it?

Thanks for reading.

-- 
Jeff Bearer, RHCE
Webmaster, PittsburghLIVE.com
2002 EPpy Award Winner, Best Newspaper Website