This has changed a bit in Windows 8.1 and can be accomplished using a RenderTargetBitmap. RenderTargetBitmap.RenderAsync(UIElement) will allow you to get the pixels for any element, including a page.
Here's an example that will save a PNG to a file specified using a FileSavePicker.
var filePicker = new FileSavePicker();
var file = await filePicker.PickSaveFileAsync();
var renderTargetBitmap = new RenderTargetBitmap();
using (var stream = await file.OpenAsync(FileAccessMode.ReadWrite))
{
var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, stream);
await renderTargetBitmap.RenderAsync(this);
var pixelBuffer = await renderTargetBitmap.GetPixelsAsync();
var pixelBytes = pixelBuffer.ToArray();
encoder.SetPixelData(
BitmapPixelFormat.Bgra8,
BitmapAlphaMode.Ignore,
(uint)renderTargetBitmap.PixelWidth,
(uint)renderTargetBitmap.PixelHeight,
96.0,
96.0,
pixelBytes);
await encoder.FlushAsync();
}