ImageView be a square with dynamic width?

The best option is to subclass ImageView yourself, overriding the measure pass:

public class SquareImageView  extends ImageView {

  ...

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    int width = getMeasuredWidth();
    setMeasuredDimension(width, width);
  }

  ...

}

The other answer works fine. This is just an extension of bertucci's solution to make an ImageView with square width and height with respect to xml inflated layout.

Create a class, say a SquareImageView extending ImageView like this,

public class SquareImageView extends ImageView {

    public SquareImageView(Context context) {
        super(context);
    }

    public SquareImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public SquareImageView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int width = getMeasuredWidth();
        setMeasuredDimension(width, width);
    }

}

Now, in your xml do this,

        <com.packagepath.tothis.SquareImageView
            android:id="@+id/Imageview"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent" />

If you need an ImageView not to be dynamically created in program, instead, to be fixed in xml, then this implementation will be helpful.

Even more simple:

public class SquareImageView extends ImageView {
    ...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, widthMeasureSpec);
    }    
}

Several of the previous answers are perfectly sufficient. I'm just adding a small optimization to both @Andro Selva and @a.bertucci's solutions here:

It's a tiny optimization, but checking that the width and the height are different could prevent another measurement pass.

public class SquareImageView extends ImageView {

    public SquareImageView(Context context) {
        super(context);
    }

    public SquareImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public SquareImageView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, widthMeasureSpec);

        int width = getMeasuredWidth();
        int height = getMeasuredHeight();

        // Optimization so we don't measure twice unless we need to
        if (width != height) {
            setMeasuredDimension(width, width);
        }
    }

}