in light of the response above.....to answer the original question.....you will need to attend a padi IDC. it is my understanding you would not have to attend for as long as say someone who is entering the course as a divemaster. but you could if you choose to.
if you are confident on your knowledge and skills then you could save some time and money by only attending the mandatory classes and then take the exam and evaluation.
if however, you have the money and the time, you could opt to join in the entire class. depends on what you want and what you can afford i guess.
i also have considered getting my padi instr so if i ever was able to retire and teach part time in a vacation destination, it would be more likely i find employment. i had decided that if i was to do this, i would take the whole course along with all the other (i assume MUCH younger haha) divers to get the full experience. i think it would also look better on a resume. it shows you were not looking for any short cuts.
for example.....when i did my evaluation etc for ssi, i attended a special clinic being offered in grand cayman. i was not obligated to attend any of the extra group meetings / seminars etc, but i did. and i learned a lot. it also showed my instructor that i was not looking for a quick certification. i was eager to take part in any extra training that i could while i was there. i think that went a long way in proving how dedicated i was to becoming the best instructor i could be.