I have great news for you, readers :)
Recently, on an internal Microsoft discussion group, I received a nice document with an overview of the data model for Dynamics AX 4.0.
After permission was received to publish this document to an external audience, I uploaded it to my personal web-site, so that all of you can download it at a convenient time.
The link is below. Enjoy!
Download AX40datamodel.doc (4.41 Mb)
UPD: A newer version of this document does not exist, so I won't be able to post it here