Active Directory is replicating only changes between DCs. This is something which every person who is working with AD for a while is familiar with. Same mechanism can be used from .NET code to request changes from DC. With .NET it's fairly easy. However AD has one more mechanisms which can be used for pooling changes from AD – less known – change notification.
This feature is in AD since beginning (at least as far as I know) however I haven't seen single application which uses this feature. Maybe because it is not very well described and only example I know was in C++ which is not very common programming language these days.
Ryan Dunn has once again provided excelent information about AD and .NET programming in his post which described different approaches to AD change tracking from .NET code. Among other methods Ryan showed how to use S.DS.P to execute extended search against AD which allows to "hook" on AD object and wait for notification when something will be changed. Nice thing to check, even if only for seeing how this thing can be done.
By the way – if You are thinking about writing some .NET application which will utilize AD or ADAM I think that it is worthy to spend some time on learning System.DirectoryServices.Protocols namespace. Maybe not as easy in use as S.DS.P but it gives You more flexibility and control over what is done in Your LDAP code.
Here You will find introduction to S.DS.P and code examples. Have a fun.