A reader pointed me in the direction of this, taken directly from the assessors website:

“If property taxpayers wish to request an individual informal review of their property’s 2009-10 assessed value after receiving the July 2009 notification, they may do so by returning a simple, printable form available on the Assessor’s website http://www.acgov.org/assessor/reassessments.htm. Property owners not in agreement with their 2009-10 assessment, who still wish to formally file an assessment appeal for the 2009-10 tax year will have from July 2, 2009 until September 15, 2009 to file an application with the assessment appeals board.”

To apply for a reassessment for the 2008 -2009 tax year, go here: http://www.acgov.org/forms/assessor/2008_2009_informal_request_decline_market_value.pdf