Hmm, the official /MP switch wasn't added in 2005? Can it really be that it wasn't added until 2008? Not even as an undocumented option in 2005? Only problem with /MP switch is that it's stored in your .vcproj file, which is somewhat unhandy when multiple developers are working on a project (ie., some on single-core laptops, some on quadcore workstations).
After googling: apparently /MP works in 2005 but was buggy. Iirc I used it without problems, though. For commandline builds, scons ftw