Replication for high availability?

[David Wright <ichbin@heidegger.rprc.washington.edu>]

(This question is unrelated to my other posts.) My LDAP server went was 
down for ~hour today, which caused chaos at my site, because it does 
login authentication. I would like to avoid such problems in the future 
by setting up a cheap box that replicates its database and could be used 
in its place. The trouble is, how do I configure my client machines to 
use the slave in the event of the master's failure? Can anyone suggest 
some neat trick involving DNS or virtual interfaces or some such magic?