I've had the Lexus infrared rain-sensing wiper option before. It replaces the intermittent mode on the stalk with the auto control, and yes, you can adjust the sensitivity just like adjusting the timing on an intermittent setting. It works by bouncing infrared light off the glass and measuing the angle at which the light beams come back to the sensor. The presence of rain drops on the glass will alter the angle at which the beam is reflected. For some reason, putting your finger on the side of the sensor box also has the same effect!
It's a cool toy, but not worth extra money IMHO. It does add to the cost of replacing the windshield, because the sensor has to be removed from the old glass and mounted on the new glass. Also, if you happen to leave it in auto mode and send the car through a car wash, it's a good way to wreck the wipers and/or the wiper motor.