At first I thought this was a joke, but apparently not: Sametime doesn’t support LDAP paging, so in very large Active Directory environments IBM tells you to increase the result page size on your servers. Haven’t they ever heard of LDAP paging?
Office 365 for IT Pros
RSS
Paul's Down-Home Page 
Yep, I just had a customer that is using Websphere. IBM recommended setting the LDAP page size to 100000. Holy cow!