First of all, if you try to install SCM on a domain controller it will fail. Why? Because the installer will go and automatically download SQL Express 2008 and install it with default options that includes trying to use a local system account, which you can't do on a DC.
Fine, I thought. I'll just download and manually install SQL Express and configure it to use a domain account. Which works just fine except that the SCM installer doesn't even bother to check to see if you already have SQL installed. It insists on installing it itself, which of course causes it to fail.
There should at least be an option during the SCM installation to either specify an already existing instance of SQL, or to configure settings such a service accounts manually.
So I'm forced to install it on a Windows 7 machine, where I've already got SQL Express 2008 R2 installed, which of course the installer doesn't care about and forces me to install SQL Express 2008.
Just.. terrible.