Small Business Expensing Temporary Expansion Act of 2008 - Amends the Internal Revenue Code to: (1) increase the expensing allowance for small business depreciable assets to $500,000 between 2008 and 2011; (2) increase the threshold for reductions in the expensing allowance to $2 million between 2008 and 2011; and (3) adjust such higher expensing amounts for inflation between 2008 and 2011.