After May 31, 2011, QuickBooks (QB) will stop providing payroll tax table updates for any users on QuickBooks 2008. This means that tax withholding and match computations with go to zero on 06/01/11 if you don’t upgrade to QB 2011.
In general, QB only supports the 3 most current versions of QB (2011, 2010 & 2009). 2008 is currently in the grace period that began in October 2010 when 2011 was officially released.
If you are not using the payroll function, then you are not required to upgrade to 2011 and you can continue to use versions prior to 2009. However, it is generally a good idea to upgrade every few years to take advantage of improvements to the programs functionality and new features the may enhance productivity.