Spectrum Relocation Improvement Act of 2008 - Amends the National Telecommunications and Information Administration Organization Act to require the National Telecommunications and Information Administration (NTIA) to post on its website detailed transition plans from each federal entity that is eligible for payments from the Spectrum Relocation Fund for costs related to the reallocation of frequencies from federal to nonfederal use. Requires the federal entities, to the fullest extent possible, to provide for sharing and coordination of eligible frequencies with commercial licensees. Requires federal entities to complete spectrum relocation within one year of receiving relocation payments.