Let's add info about author (either the whole ApplicationUser object or a userKey/userName at least) and timestamp into an event of com.atlassian.greenhopper.api.events.sprint.SprintEvent. It would be extremely useful for any custom event listener which is listening sprints updates.

Let's extend SprintEvent up to the next fields set:

public interface SprintEvent {
    ApplicationUser getUser(); // the user who initiated this event.
    Date getTime(); // the timestamp when this event is happened.
    Sprint getSprint();
}