Obstacle detection?

In my kitchen I have two small little shelves, under my Beer Tender, with four beer glasses per shelf. What I would like to do is, a LED under every glass. That makes a total of 4 LED's per shelf.

What I want to make: if you remove a beer glass from the shelf, the LED needs to turn off. If you place a beer glass on the LED, the LED will turn on.

I know how to control multiple LED's with my Arduino, but I can I accomplish the 'beer glass detection'? Maybe IR sensors?

You can use the LED itself as the sensor.
http://playground.arduino.cc/Learning/LEDSensor
www.thebox.myzen.co.uk/Workshop/LED_Sensing.html

Of course a beer glass is quite likely highly transparent to light, making a light level sensor
less than ideal here.

There are push button switches with built-in LEDs that might be fixable at the right height to
switch with the weight of a glass. In fact some keypads have soft translucent buttons designed
for going over an LED, perhaps one keypad could be split into pieces?