I have been there in July 2002 but stayed on Mabul (not Sipadan) which is 15-20 min by speed boat. I dived in Turtle Cavern together with the guide (I don't have cave certification). The dive was really great. The cavern is quite big and without certification and without dual cylinders you may visit it but deep penetration (ie the Turtle tomb itself) will not be allowed. If anyone interested I can scan the map of cavern and send it by e mail.