If its CARB certified, then its legal. If not, not.
CARB = California Air Resource Board (
http://www.arb.ca.gov/homepage.htm) = one of the most asinine institutions conceived by man to date
If you mod anything having to do with your engine/intake that gets visually inspected during a smog, make sure your mod comes with a CARB sticker indicating its been certified (it will have a certificate number on it, that your inspector can use to verify the sticker matches the mod). You'll need to affix that sticker to the vehicle so it can be read during inspection. Very few engine/intakes are CARB legal; its a very expensive certificate to buy (notice I didn't say "earn"... but I digress)
As for exhaust, you're allowed to modify your exhaust, but only from the cat back. You are also allowed to use shorty headers if they're CARB certified. Long tubes cannot be certified (due to their design), and your stock cats cannot be modified in any way (gutted, replaced with hi flow and/or non OEM spec, or deleted).
Aint California grand? :blam: