Article : Sun- and Lens Flare as a Post Process
by Charles Humphrey
Now this sample has come from my current XNA toy which is the Blacksun engine I am currently writing in XNA 3.1. It uses deferred lighting, instancing and a post processing framework that I found on a great site here, all I had to do was make it engine ready.
If you want to have a look at the effect in the engine you can see it here.
This sample is just one of the elements from the post processing framework in the engine, there are none of the other goodies in this sample, just the Sun post process. This sample has also been created using the XNA 4.0 CTP, so I only had one render target to work with at the time. This means that you can't see the sun being culled behind objects in the scene as I can't create a depth map as well as the rendered scene, to be honest, I still don't have that bit of the shader 100% anyway 
Article: Lens Flare Occlusion using Texture Masking and XNA
by Dave Charlile
Introduction
One of my side projects for the past couple of years has been a realtime fractal planet generator. It’s a huge project and not one I expect to ever really finish. But it has provided a great place for me to learn XNA and 3D programming techniques.