PiFace low voltage detector
For a brown-out type of low-voltage detect, the simplest solution would be a purpose-built device like Microchip's MCP100 or something similar. Its a little 3-terminal device which looks like a transistor but is actually a voltage detecting switch IC. You can choose one which triggers at a voltage level which suits you - there are a number of different options available.

However, if you're a bit of a masochist and really want to build something up from discrete components then try something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

D1 is a 3.9V zener, so this added to Q2's Vbe of around 0.6V means that for any supply voltage above 4.5V Q2 will be turned on. This in turn holds Q1 off and your PiFace input stays high. If the supply voltage drops below the 4.5V threshold, Q2 turns off which then allows R3 to turn Q1 on which then pulls your PiFace input low.