1. A method of evaluating a patient's condition via use of an implantable cardiac therapy device, the method comprising: sensing a patient's cardiac activity, including activity of the atria, via the therapy device; determining an interatrial conduction delay via the device; storing the interatrial conduction delay in memory of the device; comparing a record of the interatrial conduction delay over time with a corresponding threshold as a surrogate indicator of atrial fibrillation determining whether any change indicated warrants active notification of the change, and delivering a notification signal to alert at least one of the patient and attending clinical personnel of the indicated change when active notification is warranted; delivering stimulation to the heart by a lead based on the comparison of the record.