I was installing the DPM 2010 Beta (finally) and had an issue trying to get the SQL 2008 to install. Finally figured out that I had the install files stored too deeply in a network share. I figure this out by running the SQL install directly and when it when to check prereq’s it had an error on one section and when you click for more info this is what you get:
Rule "Long path names to files on SQL Server installation media" failed.
SQL Server installation media on a network share or in a custom folder can cause installation failure if the total length of the path exceeds 260 characters. To correct this issue, utilize Net Use functionality or shorten the path name to the SQL Server setup.exe file.
So, I moved it to a shorter path and it installed just fine.