Just before leaving for the holidays, the unit failed. The measured temperature started drifting upwards, more than 20 deg F at this point. Certainly much more than can be compensated for by recalibration. I don't know yet if this is a problem with the temperature probe or the controller itself but it seems more likely to be the probe. After dissecting a bit, I can see that the temperature probe is not potted in sealant as most good probes are. There seems to be a simple rubber plug at the top of the probe tube.
Interestingly, with this type of failure, the usual trick of using a controller as a backup to the thermostat on the heating rod does not work. The controller effectively shuts off the power permanently and the tank goes cold. Fortunately, this failure is slower but had it happened while I was away, the results would likely have been serious. For now, the JBJ goes on the long list of unreliable heater controllers available on the market.
Update 2013.01.08 - Ordered and received a replacement temperature probe. The guess was correct as that replacement appears to fix the controller.