The LdapConnection class is thread-safe, so you can simply reuse that on requests. The underlying Request/Reply model is layered on top of this connection. It is up to you to maintain the connection, which is your 'pooling'. For ASP.NET, this is harder to do, but for longer running apps (WPF, Services, WebForms, etc.) you just maintain an open connection and operate on it.
Ryan Dunn
Extemporaneous MumblingsThe .NET Developer's Guide to Directory Services Programming