HOT FARE! (as of 8pm Wednesday)
U.S Airways has flights between San Francisco and Puerto Vallarta for $124 roundtrip + fees, for a total cost of $228. The fare is valid now (no advance ticketing required) through February 26, 2009. Fare basis is RX0XF. Some flights are operated by United.
Plenty of availability. For example, January 9-14 is available at this price, with a connection in Phoenix.
Be sure to click on "flexible dates" option when searching for flight availability.