Description
For this project, the goal was to walk your device using a servo motor.  I tried many different times to achieve some sort of straight-walking, but since I was working alone, and with one servo motor, my best design involved the lid of a vitamin bottle that just happened to have little teeth on it that came to a point in one direction like a pinwheel.  When the servo wheel spins in that direction, the bottle top grips the table surface better than when the wheel spins back to reset itself.  The result is (mostly) forward movement.

Components Used
Breadboard, various wires, arduino, servo motor

Code

/*
 * Servo with Potentiometer control
 * Theory and Practice of Tangible User Interfaces
 * October 11 2007
 */

int servoPin = 7;      // Control pin for servo motor
int potPin   = 0;      // select the input pin for the potentiometer

int pulseWidth = 0;    // Amount to pulse the servo
long lastPulse = 0;    // the time in millisecs of the last pulse
int refreshTime = 20;  // the time in millisecs needed in between pulses
int val;               // variable used to store data from potentiometer

int minPulse = 500;   // minimum pulse width

void setup() {
  pinMode(servoPin, OUTPUT);  // Set servo pin as an output pin
  pulseWidth = minPulse;      // Set the motor position to the minimum
  Serial.begin(9600);         // connect to the serial port
  Serial.println("servo_serial_better ready");
}

void loop() {
  val = analogRead(potPin);    // read the value from the sensor, between 0 - 1024
  
  if (val > 0 && val <= 999 ) {
    pulseWidth = val*2 + minPulse;  // convert angle to microseconds
    
    Serial.print("moving servo to ");
    Serial.println(pulseWidth,DEC);
  
  }
  updateServo();   // update servo position
}

// called every loop(). 
void updateServo() {
  // pulse the servo again if rhe refresh time (20 ms) have passed:
  if (millis() - lastPulse >= refreshTime) {
    digitalWrite(servoPin, HIGH);   // Turn the motor on
    delayMicroseconds(pulseWidth);  // Length of the pulse sets the motor position
    digitalWrite(servoPin, LOW);    // Turn the motor off
    lastPulse = millis();           // save the time of the last pulse
  }
}

Images

wheel

Assembled