Phillip,
Here is the link to the current USDA regulations. The state could have some of their own, but I am not aware of them. It would seem to me for it to be diffcult for Home Depot to meet these requirements to the letter. If the nursery selling the trees to Home Depot treated them, as perscribed for movement to quarantine areas, they would have some extended protection (depending on how long HD held them) when the end customer purchased them, but I don't think if that meets the letter of the law for movement to a non-quarantine area. Basically, any citrus, or related species in a quarantine area, must be treated as perscribled before it can be move to a non-quarantine area, which in California would be any other part of the state.
http://www.aphis.usda.gov/plant_health/plant_pest_info/citrus_greening/downloads/pdf_files/federalorder-10-1-08-san.pdf
Ned