When is the best time of year to travel through California & New York? I am thinking Late September/early October '09, or maybe even April '10. Don't want to really experience the US summer season - busy!!
Also, what's the cheapest way around LA (visiting San Fran, San Diego and Las Vegas) would hire car be the go..train...bus...??
I would probably recommend September/October or April/May; basically along the lines of what you were thinking.
I agree with Wonkerer on the time to come.
I suggest you get a rent-a-car and drive yourself. It is impossible to get around LA without a car, and the drive from SF to LA (through the small towns) really should not be missed. Also, it is a short drive with a car from LA to Las Vegas (around 5 hours if it is not rush hour).
Your time periods are perfect, if you're looking for deals, stay away from the summer. And yes, you'll need a car. Fortunately the economy is creating some deals among the car rental companies. Look at a regional rental company like Fox or Advantage.
[ Edit: Sorry, no promos please. ]