According to the Palomar Airport Association:

CALIFORNIA BUDGET BENEFITS AIRCRAFT OWNERS
As part of the California budget that Gov. Arnold Schwarzenegger signed August 24, anyone who purchases an aircraft outside the state is exempt from paying California sales and use tax if the aircraft is kept outside the state for 90 days. The 90-day period had been the standard until 2004 when the state experimented with a temporary requirement that changed the exemption time frame to one year. The temporary change is expiring, and the legislature has declined to extend it or make it permanent.