From what I've read of FireShepard, it's a really bad countermeasure. Basically as described it's doing a DoS attack on people using FireSheep, probably triggered by a coding error that could be fixed.

Two big problems

1) What if the DoS affects other parts of the infrastructure like the Wireless Access Point. Can't imagine hotspot owners will be too happy if people start doing this all the time.

2) False sense of security. Using FireShepard is unlikely to stop other means of getting access to the data (eg, kismet), it only stops FireSheep (for now).