When it came time to implement this in GTC, I scoured the Cocoa Touch documentation for an easy way to crop a UIImage down to a specific size.  Apparently this was something that didn't make the cut.  Fortunately the lower level CoreGraphics methods provide a fairly simple method of doing this.  Here's what I ended up with.

- (UIImage*)imageByCropping:(UIImage *)imageToCrop toRect:(CGRect)rect
{
   //create a context to do our clipping in
   UIGraphicsBeginImageContext(rect.size);
   CGContextRef currentContext = UIGraphicsGetCurrentContext();

   //create a rect with the size we want to crop the image to
   //the X and Y here are zero so we start at the beginning of our
   //newly created context
   CGRect clippedRect = CGRectMake(0, 0, rect.size.width, rect.size.height);
   CGContextClipToRect( currentContext, clippedRect);

   //create a rect equivalent to the full size of the image
   //offset the rect by the X and Y we want to start the crop
   //from in order to cut off anything before them
   CGRect drawRect = CGRectMake(rect.origin.x * -1,
                                rect.origin.y * -1,
                                imageToCrop.size.width,
                                imageToCrop.size.height);

   //draw the image to our clipped context using our offset rect
   CGContextDrawImage(currentContext, drawRect, imageToCrop.CGImage);

   //pull the image from our cropped context
   UIImage *cropped = UIGraphicsGetImageFromCurrentImageContext();

   //pop the context to get back to the default
   UIGraphicsEndImageContext();

   //Note: this is autoreleased
   return cropped;
}

Then call the code with something like:
- (void)drawRect:(CGRect)rect
{
   //draw the whole lights off image
   //the on images will be drawn overtop
   [lightsOffImage drawInRect:rect];

   //if we don't have any lights on... no point in continuing
   if( numberOfLightsOn < 1 )
     return;

   //figure out the dimensions of numberOfLights on bulbs
   CGSize croppedSize = CGSizeMake(LIGHT_WIDTH * numberOfLightsOn, LIGHT_HEIGHT);
   CGRect clippedRect = CGRectMake(0, 0, croppedSize.width, croppedSize.height);

   //get the "on" bulbs by cropping the image
   UIImage *cropped = [self imageByCropping:lightsOnImage toRect:clippedRect];

   //create a rect to draw the newly cropped on images to
   CGRect lightsOnRect = CGRectMake(LIGHT_BULB_OFFSET_X,
                                    LIGHT_BULB_OFFSET_Y,
                                    croppedSize.width,
                                    croppedSize.height);

   //draw the "on" lights
   [cropped drawInRect:lightsOnRect];