No, they're not. The pot sets a constant voltage somewhere between two other voltages, and the PWM drives 0V and 5V pulses in rapid succession, adjusting the time period for which the signal is 0V and 5V to get an average voltage somewhere between the two.
Whether this difference is significant (and it appears to be, since the paddle isn't moving) will depend on two things: The sensing circuitry of the hardware which previously used the pot, and the filtering being applied to the PWM signal.
The sensing circuitry needs to use the same voltage levels as the PWM. 0V should be equivalent for both circuits (tied together), and the maximum voltage must be less than 5V. Your Arduino will generate voltages close to 0V and close to 5V, but not precisely those voltages - That should be OK. The Pong game may draw more current than your Arduino is capable of providing, leading to lower voltages (even constant 0V output) - That would be a problem.
Filtering is usually ignored for systems with a large amount of their own filtering. Motors have weight, and our eyes provide a high-pass filter for dimming LEDs, but this may not be the case for the input to the PONG game. Try putting a small resistor in line with the signal, and after the resistor, a small capacitor to ground. This will form a passive low-pass filter. The values used will depend on the needs of the PONG game and the frequency of your PWM (Higher frequency is better!). You can start out with a 470 ohm resistor and a 0.1uF cap. Test the output with an oscilloscope - You should see a sine wave with a small variation about the target voltage, rather than a square wave alternating from 0 to 5V. The more variation you can tolerate, the faster you can move the paddle.
However, you should also look at digital potentiometers and DACs. A digital pot functions electrically very much like a manual pot. You can move the imaginary wiper in digital steps. The MCP4131 is an example. A DAC, or Digital-to-Analog Converter, is the opposite of the ADCs available on the microcontroller. It drives a voltage, rather than averaging two other voltages. DACs are much more common than digital pots; some microcontrollers include them to complement the ADCs. I'd suggest using a digital pot in this case, because you want to replace an existing pot and don't have a lot of control over the rest of the system.
I did not figure out how to do it from teensyduino (aside from having it also be recognized as storage in addition to keyboard). But I did find out that if you skip the teensyduino and just edit C files and compile them, then manually load the hex file to the board it work just fine as a keyboard on android and PC.
Here is the page I used most to get it set up: http://www.pjrc.com/teensy/usb_keyboard.html
I'd still be very interested if any one knows how it can be set up so that it will work from teensyduino.
Best Answer
Any AVR pin function that starts with "OC" is capable of hardware PWM, therefore B4, B5, B6, B7, C4, C5, C6, D0, and D1 are all PWM-capable.