There may be a few ways to do this.
one you could ignore the voltage and just read the frequency, or the other ignore the frequence and read the voltage.
Reading the Frequency: You could pass the the signal through a diode which will then give you half of that signal.
Pretty much giving you a clock signal that the arduino can read. From there, I would create a look up table for the speeds. This of course will take some experimentation and calibration.
Reading the voltage. It has been a while since I fussed with electronics (until recently again) But here I think you would use a bridge rectifier to convert the AC signal to a DC one. Though I am not sure how you would clip the signal. And here too you would probably need to create a look up table.
Not sure if this helps, but may give you a direction to look in... or maybe a direction to look away from.http://www.arduino.cc/yabbfiles/Templates/Forum/default/wink.gif