You'll be lucky if you ever get into a taxi in Cairo that has a working meter. The meters were calibrated years ago when petrol was much cheaper, and so now everyone has to guess the price of a journey.
A good rule of thumb is about 1LE for each minute of your journey. We've been living here for four months now and haven't been shouted at by irate taxi drivers since using this rough rule.
Updates
Is this your business?
Since summer 2009 they've introduced these white cabs which have meters and air conditioning for the summer. They never seem to use it but will if you ask and they deem it hot enough. Before getting into a white cab make sure you just ask 'meter?' and point at it, as otherwise they are trying to rip you off. Prices are usually a lot less than what divers will expect in a black cab but since you know it's all above board you should be happy to round up to the nearest LE5 or 10. These taxis are a great thing to happen to Cairo, a long time coming. Black cabs are still there if you fancy the more authentic experience however.