At MaXware, we have a product that hooks into Active Directory to capture password changes. Once the change is captured, we can synch passwords downstream to connected data stores (directory, SQL DB, etc.). Like other implementations of this type of solution, ours requires having a component installed on each AD DC to identify and pick up the password changes. Although it's not generally preferable to install software on Domain Controllers, it serves a specific purpose in this case and AD administrators have been known to give-in to their business counterparts to make it happen.
The underlying need is probably obvious - multiple apps each have associated passwords. And more passwords means more to remember, less likelihood of utilizing strong passwords and higher likelihood of non-secure password practices (written notes, etc.).
As we all know, another way to solve the problem is to implement Single Sign On or Reduced Sign On solutions that enable applications to leverage a common authentication mechanism (or at least a common auth store). These solutions are complicated to implement and costly. And you usually need to find a way to synch with AD anyway since corporate users typically change their passwords using Ctrl-Alt-Delete.
ADAM directories serve as one possible solution for some of this pain. You can implement an ADAM directory to serve as the application identity store or even as a central authentication store and pass authentication request credentials through to Active Directory. This means a single password stored in Active Directory that can serve the network environment as well as the apps that leverage ADAM as the Auth Store. If you centralize the store, you don't really NEED to centralize the authentication mechanism, which might be nice in some cases. In environments with a large amount of custom-developed applications, a centralized mechanism would probably provide significant value even if there's already a common authentication store.
Wouldn't it be nice, though, if you could do the same thing without needing to implement ADAM directories and without storing the AD password in two locations? What if a Virtual Directory could pass authentication requests to Active Directory the way that ADAM does? You could potentially use the Virtual Directory to expose only specific attributes to given applications, reduce the overhead of an additional directory infrastructure, eliminate the need to store the AD password in multiple locations (and the need to install agents on AD DCs) and still achieve authentication to multiple systems using a single set of credentials.
[UPDATED Mar. 8, 2006] - Shorty after posting this, I was informed that MaXware Virtual Directory (along with its competition) does, in fact, support passthrough authentication to AD. I guess I thought ADAM was doing more than a simple bind, which is not the case -- I think I learned that years ago and forgot it again. So, I removed the question on technical feasibility. I guess the question becomes why aren't more people talking about this? It seems like it would be an incredibly useful application of our product.
[UPDATED Mar. 18 2009] - I posted more information about ADAM passthrough authentication.