Homeowners Insurance and Mitigation Assistance Act of 2008 - Amends the Internal Revenue Code to allow tax credits for: (1) 50% of the increases in homeowners insurance premiums for taxpayers whose principal residences are located in certain federally declared hurricane disaster areas or in states which border the Atlantic Ocean or the Gulf of Mexico, up to $500 annually; and (2) 25% of the qualified hurricane, earthquake, and tornado mitigation property expenditures of homeowners and businesses, up to $5,000 annually. Includes within the definition of "qualified hurricane, earthquake, and tornado mitigation expenditures" expenditures for property to improve the strength of a roof deck attachment, create a secondary water barrier, improve the durability of a roof covering, reinforce the connection between a roof and a supporting wall, and protect exterior doors and garages.