Unfortunately, there is 100% truth to this.  DNN does not facilitate this adaption of login.  It must be changed to do so.

Having done this myself, I can tell you that in order to do this right, it requires both changing the core code of DNN, and potentially writing custom Authentication, Profile, and Role providers to facilitate and handle any unwanted or unknown exceptions in work flow.

* EDIT *  I almost forgot...  In order to provide a favorable end user experience, there are several places in the localization files that would need to be updated and maintained during each upgrade as well as the previously mentioned providers and code changes.  The localization changes are mostly to make sure that where the term "User Name" is, it is changed to "E-Mail Address".  That is really only a usability update.

Jeff, unless you do not want to handle the ways that the end-user can get around being forced to use their e-mail address and to also facilitate the anonimity of a username vs. e-mail address, you will need to make changes in the core and providers to switch the expected username to the Display Name, and the e-mail address to the username.  Not to mention those items in my edit of my previous post.