This has sort of been fixed in 7.5. We have our Macs bound to AD and what happens is, when a client goes to our SD page, a window pops down from the top of the browser asking for username and password. This is separate from the actual login on the page. Our clients can then enter their username and password (as requested) but note, not their email address. Once they click OK they can get past the login page and are into the application.
The issue we see we this is that on some of the sub-windows such as when creating an incident, for the first time, our techs also have to login there with their email and password. It seems that the credentials aren't getting passed along for some reason. If they had canceled the original drop-down authentication box and opted to log in with their email address and password then they don't get prompted to log in again later on. It seems that credentials are handled well one way and not the other. This could be, however, because of the way we have things set up by fronting our SD server with an F5 load balancer. I just haven't had time to troubleshoot it.