Interesting - I'm curious if I could get these to work with the latest revision Raspberry Pi boards that will be shipped soon (I only ordered them half a year ago... But they got upgraded to the newer 512MB ones, so I suppose it's OK...). I'm far from satisfied by the PWM sound experience, and I don't want USB to fire any more needless interrupts... I2S sounds like a proper solution (Yay for the addition of full I2S to the GPIO on rev2 pi's <3)
Emphasizing that measuring mains is dangerous is okay when we're dealing with newcomers to electronics, but once you reach a certain stage (Such as the ability to see what the hell is going on on that breadboard), it's time to realize that you need to work with it every now and then. Just don't end up with the phase in one hand and neutral/another phase in the other. It hurts, as I have concluded through numerous *cough cough* scientific studies of the subject. (Especially phase-to-phase. 400VAC, surprisingly enough, makes you a little shaky afterwards.)
But, seeing you need some serious input ranges (0-700v peak-to-peak, assuming we're on European mains like me, or ~350v max if you're on American/Japanese/xyz soil.), what about a long resistor ladder with something like zeners/comparators and fast transistors to "select" a voltage scaling on the ladder? You might want to "hold" the resulted scaling, to avoid confusing your frequency counter with sudden scaling. Also, you could probably "isolate" the main circuit from this scaler with optocouplers (high linearity ones), in which case the worst thing that could happen is the sudden death of an optocoupler.
Hope some of my ideas are even slightly useful, and try not to have both of your hands in the project when connected to mains. It only hurts/fries nerves if it's one hand, but two can kill. :)
While possible, you need to be aware of how X10 works - It doesn't just use the mains as a USART, 1-Wire or I2C line - Assuming I recall correctly, it's basically a RF setup using the mains as signal path, meaning you will need an X10 interface - At which point, the interface would present you with something simple, and the only thing left is to read/write to it with the bus pirate.
