If they are a developed Hacking Group, they might have left a Logic Bomb on the Server that your Forum was hacked in, So your Forum, as well as the server its on, could possibly be compromised at a later date (Whenever they want) So be careful and look for anything Unusual (Though If they are that good I doubt you will find it, at least not right away) So good luck...
No wonder why it gets hacked; IPB 2.1.3! There are sooo many PUBLIC SQL Injections availible! Why you never upgraded? I hope you got backups dude. I've added you on msn, we'll talk further.