All of the samples I cam across using WriteableBitmap seemed to be from a prerelease version. They call the constructor with an additional parameter and don't call Invalidate() which is required for the anything to actually show up in the bitmap.

private void UpdatePreview()
{
WriteableBitmap bitmap = new WriteableBitmap((int)imageSource.Width, (int)imageSource.Height);
bitmap.Render(imageSource, new TranslateTransform());
bitmap.Invalidate();
imgPreview.Source = bitmap;
}