I had a small problem with collision detection starting out with XNA 4.0 so I figured I'd release a simple snippet that helps with two different types of collision detection; Per-Pixel and Rectangular.
To use simply copy and paste the snippet into your current source, or you could put it into an application extension so you don't have to find the source code over and over again.
I'm developing a beginners developer kit for XNA 4.0 for those who are interested in XNA development but don't know quite where to start. There are several game helpers as well as some useful methods for XNA Development. The most useful being some physics methods; motion (movement on x and y), jumping (simulation of fake gravity), drag, heat, and much more; as well as some useful methods for AI; for example, Bezier curves, pathfinding, decision simulation, button combos, so on and so forth. So you can look forward to that if you're interested in learning XNA. I will include the kit with a simple showroom style app/game that will print source code, show debugging values, and what all is going on in each area. It includes 9 basic games to play around with, and learn how to make. It will be freeware so that's a plus as well. :)
Hope it helps,
Jamie - Studio 41 Games (Owner / Head Developer)