Research and Development Tax Credit Improvement Act of 2008 - Amends the Internal Revenue Code to revise the tax credit for increasing research activities by: (1) phasing in increases in the alternative simplified tax credit rate through 2009; (2) establishing a 20% alternative simplified tax credit rate in 2010 in lieu of the standard research tax credit rate; (3) increasing the amount of basic and contract research expenses eligible for such tax credit; and (4) extending such credit through 2012.