This is how I obtained a non-contact rotation speed reading using a very inexpensive photo tachometer (in the USA, you can get this to your door for under $30). I purchased the Neiko branded tach from Amazon and I am sure it can be found elsewhere and under different or even no brand label. Mine uses 3x1.5 AA batteries for an operating voltage near 5V. The interface is very simple.
+5V from Arduino - connected to the circuit side of the "Test" button on the tachometer (circled RED in the picture)
GND from Arduino - connected to the battery negative in the battery tray on the tachometer (circled Yellow in the picture)
SIGNAL to Arduino - connected to pin 12 of the micro on the Tachometer which is an interrupt-enabled pin on the micro in the tach and is fed from an HC4050 Buffer chip
The cable used is a shielded headphone extension cable with TRS (tip ring sleeve) connectors. The shield in this cable has a braided wire that I used for ground. I used the Black (Left Audio) for the signal and the Red (Right Audio) for +5V. The tachometer body also luckily had a plastic plug in the side for some non-existing feature. I drilled 3 holes in this piece to get a tie location for a zip-tie fastener and of course a hole to bring the cable through.
On Arduino I used attach interrupt with FALLING option.
Here are some picture thumbnails from photobucket. If you click the picture you should be able to see a large version. I am sorry about missing a picture of my connection to the button. On the tach, the battery is normally an open circuit and when you press the "Test" button it closes the circuit to power up and take readings. I connected +5V to the other side of the switch so when Arduino is connected the tachometer is always powered.
I know that a completed project could go into the exhibition forum, but this project is a complete sensor rather than a complete Arduino solution. It could also be considered a hacked object but this is so simple I don't think it quite qualifies as hacked. One thing to look out for; while my tach is powered by 3 1.5V cells, I saw one described as powered from a 9V battery. If the 9V device is not regulated to 5V, then you would need additional components to do the interface but it should still be quite easy.