I finally got this to work in asp.net.  
There are actually 2 ways to do it.

The first method is the simplest, using the full path to the domain.   This
method will not work if you try to point to a specific AD container, using
paths like CN=x, DC=y, Dc=z OU=a etc.

       sADPath = "LDAP://sdp-dc3.sdptech.com"
       sADUser = "adminguy"
       sADPassword = "adminpwd"
       oDirectoryEntry = New DirectoryEntry
       oDirectoryEntry.Path = sADPath
       oDirectoryEntry.Password = sADPassword
       oDirectoryEntry.Username = sADUser
       oDirectoryEntry.AuthenticationType = AuthenticationTypes.Secure

so, how do you get to a specific container to query it?  
You need to use the DirectorySearcher object and filter - I wont go into that
here, there are many examples of using the DirectorySearcher on the web

Anyway, you filter it such as:

(&(objectClass=user) (objectCategory=Person) (!(telephoneNumber=*)))

OK, the second method is a bit tricker, but allows you to use container
pointers in the LDAP declaration.

       Dim oDirectoryEntry As DirectoryEntry
       Dim sADPath As String

       sADPath = "LDAP://DC=sdptech,DC=com,CN=Users"
       oDirectoryEntry = New DirectoryEntry
       oDirectoryEntry.Path = sADPath

if you try running this, you get errors.  You need to set up IIS and also the
web config file as follows:

In IIS, navigate to the Virtual directory for the web application
In the console, select the aspx page that runs the code that accesses the
Active Directory.
Click "Properties" from popup menu, then select the "File Security" tab.
Click the "Edit" button to display the "Authentication Methods" form.

Ensure that "Integrated Windows Authentication" is checked.
Ensure that "Anonymous User" is checked.
All other options should be unchecked.

In the "Anonymous User" section:
select a DOMAIN user name (such as domain administrator) and user password
ensure that "Allow IIS to control password" is unchecked.

Finally, in your application web config file, ensure you have the following
entry:

   <authentication mode="Windows" />
   <identity impersonate="true"  />