We have a 2004 Prius with 103,000 miles and Love it.
This week we had our first problem. The "Hybrid System warning light" came on along with the word "Problem" in red on the dash display. The car seemed to be running fine. We drove less then a mile and parked.
We had a flatbed pick it up and bring it the 70 miles to our dealer. They checked it out and found two things for a grand total of $80.00.

What they said was the ECM (engine control computer) needed to be reprogramed with two updates. One was coved under warrenty (since they did not do it at our last service visit, 98,000 miles) and the other was not.
This is what the invoice has on it.
"105242 Code P3191 Eng does not start Several updates for ECU 1st being 50P also Recalibrated ECU not under warrenty."

Anyone have any thoughts on why they needed to update the ECM?