We have published a topic about upgrading domain controllers to Windows Server 2012: http://technet.microsoft.com/library/hh994618.aspx.
It covers a lot of useful background information to help you prepare for an Active Directory upgrade, including system requirements, supported upgrade options, functional level features and requirements, application compatibility, known issues, and more. We will keep updating the topic as relevant hotfixes get released to help organizations be better prepared. Let us know what other topics related to upgrading DCs that you want to see covered.
Special thanks to Arren from MS support for writing much of the content for it.
And while I am on the subject, if you are upgrading to DCs to Windows Server 2008 R2, see http://technet.microsoft.com/en-us/library/upgrade-domain-controllers-to-windows-server-2008-r2.aspx. This is another topic that covers many similar issues about upgrading Active Directory, and shares a lot of experiences from Microsoft support organization to help organizations avoid problems related to upgrade.
Justin [Active Directory Documentation team]