Last year I was looking for some ultrasonic range finders for our bot and I through a response on chiefdelphi the LV-MaxSonar was recommended to me. It outputs the range to an object in several ways including serial, analog voltage, and pulse width. Of these methods I would probably choose the analog voltage since you only need to multiply this by a constant to resolve the range. The distance calculation is already done for you!
here is the website for the sensor.
http://www.maxbotix.com/
Hope this helps!