We're having trouble getting a new Windows 7 Ultimate machine joined to an OS X domain.
I found this Apple KB article discussing the very same issue we're having.
Basically, when entering all the information to join the domain (which all works fine on XP and Vista) we're receiving a "domain does not exist or could not be contacted" error message.
I also changed two local security policy settings per this article that are known issues between Windows and SMB authentication.
Has anyone else successfully put a Windows 7 machine on an OpenDirectory domain?
In my experience, it's not possible. Instead, I'am currently trying to build my own version of Samba and to get it to play nicely with OD without all the Apple patches to Samba. That's difficult as well and might have some limitations, but at least it might be doable.
According to some sources, an update to Samba on OS X Server that works with Win7 might never come because of licensing restrictions following the adoption of the GPLv3 by the Samba team.