New KB article came out few days ago regarding possibility of backup failure on Windows 2003 SP1 DCs in specific configuration. Specific configuration means different location for neither DIT file or database logs file if one of this elements is located also on the system drive. So both (DIT and logs) on different drives are good, unless one of these drives is a system drive.
More infromation at a KB909265 source.
Thanks for Susan Bradley for pointing out this KB article on ActiveDir.org mailing list.