I see that we released the Microsoft Active Directory Topology Diagrammer today. I'm working from home today, so I can't run it in AD and attach a screenshot, but this should be helpful for documenting your AD infrastructure. According to the product description:
The Microsoft Active Directory Topology Diagrammer reads an Active Directory configuration using ActiveX Data Objects (ADO), and then automatically generates a Visio diagram of your Active Directory and /or your Exchange 200x Server topology. The diagramms include domains, sites, servers, administrative groups, routing groups and connectors and can be changed manually in Visio if needed.
With the Active Directory Topology Diagrammer tool, you can read your Active Directory structure through Microsoft ActiveX® Data Objects (ADO). The Active Directory Topology Diagrammer tool automates Microft Office Visio to draw a diagram of the Active Directory Domain topology, your Active Directory Site topology, your OU structure or your current Exchange 200X Server Organization. With the Active Directory Topology Diagrammer tool, you can also draw partial Information from your Active Directory, like only one Domain or one site. The objects are linked together, and arranged in a reasonable layout that you can later interactively work with the objects in Microsoft Office Visio.
This feature used to exist in Visio many years ago, so I'm glad it's available again (even if not built directly into Visio). Actually, I see that this is version 2.0.2745. Maybe it previously existed and I just didn't know about it. Hmmm... Try it out and let me know how well it works!
*Update: Björn has posted some screenshots on his blog.