When adding a new task from an incident (using the add new task button on the OOTB toolbar) the task priority can be set from the incident Priority and also be made so that user changes during the initial task creation are saved rather than overwritten by the automatic update.
This can be achieved by adjusting the relationship properties between incident and Task.Assignment:
1. Go to Configure Application
2. Go to the Incident Business Object
3. Go to relationships and find the relationship "IncidentAssocTaskAssignment"
4. Add an update field entry under the "Update Task.Assignment's fields" section at the bottom
-Set field: Priority
-To this object's field value: Priority
-Leave "Only when field is empty" UNchecked
-CHECK "Overwrite defaults, but preserve user edits"
The update criteria will make the new task creation dialogue have the Priority of the Incident it is being created from.
The last checkbox setting will allow any user changes to save. If it is not checked you will find user changes during the creation of the new Task will revert to the Priority of the incident.