Adobe has begun to enforce the licensing requirements for running AfterEffects on a render farm. According to a posting on Adobe's forum, this policy has been in effect for some time, but they've just begun to enforce it now.
It appears that AE users will be required to purchase additional AfterEffects licenses in order to use aerender on a render farm.http://forums.adobe.com/message/3925384?tstart=30
If I read this correctly, it seems that a AE user would need a license for every 2 render nodes. Not sure if the cost works out compared to Adobe's "volume licensing".