I think the Texas Instruments website could be worth a look - I'm sure they do some micros specifically for power meters, so they might well have some info on the general principles.

The original post didn't specify that this was "high" voltage - just 50Hz.
So the optos may not be needed. (even if it is 220V, you might not need 'em if you're very careful)

I suppose it might also be possible to measure the current with a Hall-effect device?