I experience the same problem as Lyceum, though after starting by means of a reset, the bottom sensor still doesn't work. That is why I think the sensor might be broken. However the weird thing is that after a charge, the sensor sometimes works again, in which case I can also start up in the normal way.
It would help if you could assign the functionallity of the bottom sensor to one of the side sensors, however I have never heard of this option.
I have experienced the problem in both 1.6.5 as well as in 1.7.1. I have not (yet) intalled 2.0. I was planning to install this once the final version would be available (which might never happen now).