I'm a fan of the AEM Brute Force. They should be CARB exempt or at least certain models are according to their website. The performance gains are about the same, maybe a little more, than the K&N. Figure about 8-12 hp -- no tuner needed.
My draw to the AEM was their "Dryflow Air Filters" and polished intake tube. The Dryflow never needs oil which usually = less down time between cleanings and zero chance of over-oiling.
From what I can tell by checking the CARB website (http://www.arb.ca.gov/msprog/aftermkt/devices/amquery.php), K&N is the only street legal intake mod. Currently it's only certified for the '05, but given the fact the '06 is basically the same car it should pass when the results come out next March.
The CARB certified part # is 57-2566. The K&N part # if you get the uncertified part would be 63-2566 (you can then contact K&N to get a CARB sticker)
I also saw the AEM filter listed at http://www.rpmoutlet.com/05v6aem.htm and that web page says it's CARB exempt, but I checked the CARB website under air filter/instake modifications and it isn't listed.