Using the super awesome TouchOSC app for the iPhone connected to Processing for converting Open Sound Control signals into serial commands send out via USB to an Xbee...I am able to drive a small tank.

The tank is entirely made from SparkFun parts:
Arduino
Ardumoto
Dual Motor GearBox
Stackable Arduino Headers
Super Awesome Tank Treads
LiPoly Battery Charger
Batteries
Small Solar Panel
XBees
XBee USB board
XBee Explorer board
Also used a number of misc jumper wires, rubber bands, a block of wood and a few small pieces of aluminum (from the back of a PC).

Of course you will need an iPhone to get everything running exactly as I have setup, but you don't need to use an iPhone to control your tank, you could use a midi controller, a multi-touch screen, a random hardware controller, or really anything that can send serial, OSC or anything into Processing.

Here is the Source Code:
Source code for Arduino is from a Mechomaniac.com:

//-----------Start Arduino code ---------------

// Test program for SparkFun Ardumoto board
// Copyright (c) 2009 mechomaniac.com
 
// To use, connect the Arduino to a computer and send commands using a serial terminal.
// eg AR40#   motor A forwards with a speed of 40
 
#define PwmPinMotorA 10
#define PwmPinMotorB 11
#define DirectionPinMotorA 12
#define DirectionPinMotorB 13
#define SerialSpeed 9600
#define BufferLength 16
#define LineEnd '#'
 
char inputBuffer[BufferLength];
 
void setup()
{
  // motor pins must be outputs
  pinMode(PwmPinMotorA, OUTPUT);
  pinMode(PwmPinMotorB, OUTPUT);
  pinMode(DirectionPinMotorA, OUTPUT);
  pinMode(DirectionPinMotorB, OUTPUT);
 
  Serial.begin(SerialSpeed);
}
 
// process a command string
void HandleCommand(char* input, int length)
{
  Serial.println(input);
  if (length < 2) { // not a valid command
    return;
  }
  int value = 0;
  // calculate number following command
  if (length > 2) {
    value = atoi(&input[2]);
  }
  int* command = (int*)input;
  // check commands
  // note that the two bytes are swapped, ie 'RA' means command AR
  switch(*command) {
    case 'FA':
      // motor A forwards
      analogWrite(PwmPinMotorA, value);
      digitalWrite(DirectionPinMotorA, HIGH);
      break;
    case 'RA':
      // motor A reverse
      analogWrite(PwmPinMotorA, value);
      digitalWrite(DirectionPinMotorA, LOW);
      break;
    case 'FB':
      // motor B forwards
      analogWrite(PwmPinMotorB, value);
      digitalWrite(DirectionPinMotorB, LOW);
      break;
    case 'RB':
      // motor B reverse
      analogWrite(PwmPinMotorB, value);
      digitalWrite(DirectionPinMotorB, HIGH);
      break;
    default:
      break;
  } 
}
 
void loop()
{
  // get a command string form the serial port
  int inputLength = 0;
  do {
    while (!Serial.available()); // wait for input
    inputBuffer[inputLength] = Serial.read(); // read it in
  } while (inputBuffer[inputLength] != LineEnd && ++inputLength < BufferLength);
  inputBuffer[inputLength] = 0; //  add null terminator
  HandleCommand(inputBuffer, inputLength);
}

//----------- End Arduino code ---------------

Processing code:

//----------- Start Processing code ----------------

import oscP5.*;
import netP5.*;
import processing.serial.*;
Serial arduinoPort;
OscP5 oscP5;

float [] fader = new float [3];

void setup() {
  oscP5 = new OscP5(this,8000);
  arduinoPort = new Serial(this, Serial.list()[0], 9600);
}

void oscEvent(OscMessage theOscMessage) {

    String addr = theOscMessage.addrPattern();
   
       if(addr.indexOf("/1/fader") !=-1){
       String list[] = split(addr,'/');
     int  xfader = int(list[2].charAt(5) - 0x30);
     if(theOscMessage.get(0).floatValue() !=0){
     fader[xfader]  = theOscMessage.get(0).floatValue();
     }  
    }
}

void draw() {
//---------------------------------Motor A
 if(fader[1] > 0.65){
    arduinoPort.write("AF100#");
  }
   if(fader[1] < 0.35){
    arduinoPort.write("AR100#");
  }
  //--------------------------------Motor B
   if(fader[2] > 0.65){
    arduinoPort.write("BF100#");
  }
   if(fader[2] < 0.35){
    arduinoPort.write("BR100#");
  }
  //----------------------------stop commands
     if(fader[1] < 0.65 && fader[1] > 0.35 ){
    arduinoPort.write("AF0#");
  }
       if(fader[2] < 0.65 && fader[2] > 0.35 ){
    arduinoPort.write("BF0#");
  }
 
}

//---------------------- End Processing Code ------------------