I was reading up on USB connectors on Wikipedia and I've noticed that the contacts for the data lines (D+ and D-) are shorter in both the Type A and Type B plug.
What could be the reasoning behind this design?
Hot plugging. You want the power supply to be connected before the datalines. Some chips may latch if a signal is supplied to the signal pins before power is applied. This means the chip will internally short out and may get destroyed when subsequently power is applied.
In the old days you had to power off you computer before you could add a peripheral to it, that is not necessary with USB (and some other interfaces).
The reasoning behind this design is to always apply power prior to data so that bus-powered devices can interact with the host when connected. A USB device must react to bus reset 10 ms after being plugged in.