They used to is the keyword. They used cos they used to be using an older version of Opticodec which had the mime-type as:
That has since been updated to:
as well as Orban proposing the mime-type be standardized.
Oddcast and AudioCoding.com projects also use audio/aac. I'm not sure about quicktime though... They always seem to be doing thier own thing.
I would recommend sticking with audio/aac like everyone else, and you will help put the presure on the non-supporters of this. The real company that benefits by us becoming scattered is evil evil ms.