From ANIMAL PEOPLE, September 2006:

SACRAMENTO–The California legislature on August 14, 2006
sent to Governor Arnold Schwarzenegger a bill to criminalize leaving
pets unattended in weather that puts the animals’ health at risk–but
specifically exempted “horses, cattle, pigs, sheep, poultry or
other agricultural animals in motor vehicles designed to transport
such animals,” a clause excluding from protection more than 99.9% of
all the animals who die in transit from either excessive heat or cold.
Violators of the California bill could be punished by fines
of up to $500 and up to six months in jail. The bill specifically
empowers animal control officers to break into cars to rescue animals
in distress.
But Virginia Handley of Animal Switchboard, the senior
animal advocacy lobbyist in California, did not join other humane
leaders in claiming an apparent victory. She pointed out that many
California agencies have already successfully prosecuted people who
left pets in hot cars under the state anti-cruelty statute–which
permits stiffer penalties.

Read more