Currently, with JIRA Agile, sub-tasks can only be tied to one parent and that parent can only be assigned to one Sprint.

The ideal situation would be to include the ability to either assign the issue to a particular Sprint (currently supported) OR allow the issue to "Inherit Sprint from Sub-Tasks" (proposed new option). Also, if the "Inherit Sprint from Sub-Tasks" option is selected, the sub-task not the parent issue is included on the Agile board. The Parent issue simply keeps record of the progress of all subtasks.

With this option, each sub-task could be assigned a Sprint. The estimation added to the Subtask would then be rolled up to the Parent based on the Sprint the "In Progress" subtask is currently assigned. This would allow Parent issues to be tracked across more than one sprint.