Years ago LDAP replication technology removed the limit to the number of LDAP reads your directory service can support. Need more read capacity? Simple, add another LDAP server instance. What many might not know is that since March 2007, the data distribution capability in Sun's Directory Proxy Server has made it possible to scale LDAP write operations, too.
As described in the Deployment Planning Guide section on data distribution, simply adding more Directory Server instances to a replicated topology lets you scale reads, but not writes. Why?
Replication brings you high availability by copying data to each server in the replication topology. If one server goes down, others that remain up continue providing LDAP directory services. With a bit of load balancing, your LDAP applications hardly need notice that a server instance has failed. Multi-master replication means this high availability technology works for writes, too. As with other high availability technologies, you gain availability by adding redundancy.
Replication provides its function by replaying changes from each write operation onto every server involved in a replication topology. As a result, the number of operations to perform for each write (add, delete, modify, modDN) is equal to the number of servers involved. So you may have N servers to do the work, but for each write there are N writes to replicate! The writes are only fully completed across the replicated topology as fast as the slowest replica can finish its updates.
When you need go beyond hundreds into thousands of LDAP write operations sustained every second, the way to do so today is not by adding more servers to your existing topology. Instead you split your topology up into chunks, distributing a part of your data set onto each chunk. For high availability, each chunk is its own topology of replicated Directory Servers. Since version 6.0, Directory Proxy Server allows you to split entries from your directory service across chunks. The example shown in the documentation depicts three chunks, the first with surnames (attribute: SN) starting with A through H, the second for I through P, and the third for Q through Z.

The data view shown in the diagram is a configuration concept from Directory Proxy Server. Notice there is a special chunk for the top of the directory information tree.
Now, with three separate chunks, you can achieve as much as three times the write throughput. How? Assuming the write operations are equally distributed, each write has only one chance in three of involving a particular chunk. Each of the three chunks can process write operations in parallel up to the maximum number of writes each chunk can handle. The number of writes you can perform is limited only by the number of truckloads of hardware you can afford. (Maybe I should have mentioned this before you got everything you wanted for Christmas. ;-)