Installing SQL Server on a Domain Controller is not recommended, but can be done. But, Installing SQL Server on a Windows Server 2008/2008 R2 Read-Only Domain Controller (RODC) is not supported, and not possible at all.

It is fully documented in SQL Server 2008 Books Online (BOL):
Installing SQL Server on a Domain Controller (at the bottom of the page)
http://msdn.microsoft.com/en-us/library/ms143506.aspx

“SQL Server Setup cannot create security groups or provision SQL Server service accounts on a read-only domain controller. In this scenario, Setup will fail

And for SQL Server 2005: in the following Knowledge Base article:
http://support.microsoft.com/kb/936302 (Problem 4)

If you try, you will get a similar message: “Failure creating local group…”