[stevem]

Here's the response I just got from Bangkok Airways head office. Appears all depends on the airline that you are flying into Bangkok on. If your airline can/does check your luggage through to Phuket, then you can clear immigration/customs in Phuket, otherwise it must be done in Bangkok. Looks a case by case situation.

Thank you for your interested in Bangkok Airways's service.

According to your e-mail, first of all, could you please kindly check
with the airline operates your international flight whether they can
check through the baggage for Bangkok Airways or not. Due to this
depends on the carrier operated your original flight which will have the
baggage check through agreement with us or not. Also, please be informed
that our check baggage allowance is 20kgs per person.

By the way, the imigration and custom clearance can be done at Phuket
International Airport in case of baggage check through.