If the "Microsoft SQL Server 2008 Native Client" from the "Microsoft SQL Server 2008 Feature Pack, August 2008" (http://www.microsoft.com/downloads/details.aspx?FamilyID=c6c3e9ef-ba29-4a43-8d69-a2bed18fe73c) is already installed on a machine, SQL Server 2008 setup will look on the installation media for a file with the same name as the downloadable version (e.g. "sqlncli10_x64.msi") and fail when it cannot be found (since the version on the installation media is named, in this case, "sqlncli.msi").
Other than the different file names, the Feature Pack "sqlncli10_x64.msi" file and the SQL Server 2008 installation media "sqlncli.msi" file are identical.
It appears that this may be related to the fact that the file name of a Windows Installer package cannot be changed without also changing the product code, as discussed at http://msdn.microsoft.com/en-us/library/aa367850.aspx and http://blogs.msdn.com/robmen/278746.aspx.
This is likely reproducible with the other processor architecture versions of the Native Client and SQL Server 2008, since their file names also differ between the Feature Pack and the SQL Server 2008 installation media. It may also be reproducible with other redistributable components as well (the "Microsoft SQL Server System CLR Types" and "Microsoft SQL Server 2005 Backward Compatibility Components", available from the same Feature Pack, appear to be likely candidates).