I know that in CiviCRM 4.6, the ability to send scheduled reminders to people based on a custom field (e.g. marriage date) was added. However, I'm unsure if it's possible to send a scheduled reminder based on the custom field of an ACTIVITY. I couldn't figure out how, if so.
The use case is to have a custom activity field labeled, "Remind me on X date", which is similar to functionality present in Raiser's Edge.