Howto - Make a Wii sensor bar with bubble gum, a piece of string and tinfoil.

Well not really. But you'll need an old mouse, some USB cabling and 20 cm of cat5 wiring.

Start with a generic mouse:

Most logitech and clones are built around a 20pin DIP chip surrounded by miscellaneous bits n bytes.
On this particular circuit board the two LED/sensor pairs are labeled PT1 and PT2. The dark components are sensors, the clear ones are the infrared LEDs which interest us. Next to those are resistors. They should be rated appropriately for +5v (about 20-40 ohm) coming from the PS2 cable and there should be one for each LED. We need those ones too.
Once we've liberated the LEDs and resistor from the circuit board, we want to build this simple circuit:

Now it's time to butcher the USB cable. Snip off the 'B' part of the USB cable, we want to keep the 'A' one that goes to the computer. Strip the end of the cable to reveal the 4 wires, red, white, green and black. We want red (+5v) and black (ground). Strip the 20cm cat5 keep two pairs of wiring say, orange/white and brown/white.
Solder the two resistors to the red lead of the USB cable, then solder the orange and brown leads to the other end of the resistor. Solder the white-color-striped wires to the black USB wire.
Now comes the tricky part. If the LED polarity are not clearly indicated, you'll have to figure out which way to hook up things. But sadly, you are not a superhero and cannot see into near infrared.
That's ok, because Q-Bert just got an X-Box webcam a few days ago. Borrow his and point it at your newly assembled circuit. You should see the LED shining. If not, swap the connection on the LED and try again. Q-Bert won't lend you his Webcam? Fine. Sheeesh. Get a camera phone, a digital camera... anything with a CCD will see the infrared LED.
OK. Are you done yet? Good. Now find your favorite method of encasing the LEDs and cabling-- LEGOs will do fine. I have mine wrapped in some transparent tubing.
Enjoy your new useless gadget. And remember: If it hurts when you Wii, that means you are doing it too much.