The Windows Phone 8 SDK is now available and with that WriteableBitmapEx was updated too. The performance of the WinRT XAML version was also improved dramatically and an update is highly recommended.
WriteableBitmapEx is available for 4 platforms: Windows Phone 8 and 7, WPF, Silverlight and Windows Store WinRT .NET XAML.
You can download the binaries here or via the NuGet package. The packages contain the WriteableBitmapEx binaries. As usual all samples and the source code can be found in the repository.