I had that exact same thing happen to me in real life, so I did some checking... and I think the official rule is something like, the ball must come to rest below the top of the cup to be considered in the hole. This is not a direct quote because I can't remember the wording. It really sucked cuz it would've been my first ever eagle... I'm not very good in real life so that would have made my decade for sure! I'll try to find the official rule...
Lee
*Edit* Taken from usga.org under Section II Definitions:
Holed
A ball is "holed" when it is at rest
within the circumference of the hole and all of it is below the
level of the lip of the hole.