Our LDAP connection runs over SSL, but to do this we need access to certificates on your server. Navigate down to c:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys.
NOTE: The Application Data folder and its child objects are all hidden by default and you will need to change your viewing settings to see them.
Make sure that Network Service has permissions to read everything inside of the MachineKeys folder and then up the chain.
Windows will cache the LDAP connection so you may also need to change your web.config LDAP string to use IP addresses instead of DNS.