I'd go with simple 434 MHz Rf modules with standalone arduino ('328, 16 MHz xtal, two 22 pf caps, 10k resistor, two or three 100nF/0.1uF caps, whatever resistor/LED you need, 5V wallwart for power) at each desk, and matching Tx module whereever it goes.
RF module examplehttp://www.robotshop.com/productinfo.aspx?pc=RB-Ons-02&lang=en-US
You have 20 IO pins. Need 2 committed for Rx/Tx. (D11, D12 are default when using VirtualWire library).
Perhaps arrange pins as headers for programming: 5V, Gnd, Reset, SCK, MOSI, MISO, for ICSP. Or 5V, GND, GND, DTR, Tx, Rx, FTDI.
Arrange other 7 other as 'address' pins so can select 1 of 128 address.