Just tried to access my Office 365 tenant using the new Windows Azure Active Directory Manager (WAADM) that was recently improved further.
Bear in mind, this is possible because the Office 365 tenant when provisioned actually is created in Windows Azure Active Directory (WAAD). But WAAD is not only for Office 365, you can also configure WAAD to integrate with your on-prem Active Directory without having Office 365 in the mix.
You can also created dedicated domain controllers in Windows Azure that integrates with your on-prem AD meaning you for instance could use it as a disaster recovery site. For more info see: http://www.windowsazure.com/en-us/manage/services/networking/cross-premises-connectivity/
Must say its pretty darn cool! Try to think about the perspectives here. Before we know of it, we no longer need an AD on-prem but will migrate this to WAAD.
First, we move your applications and then we take the AD! 
Read more about the WAAD related improvements here:
http://blogs.msdn.com/b/windowsazure/archive/2012/09/12/more-advances-in-the-windows-azure-active-directory-developer-preview.aspx
Until later,
Henrik Walther