I found a need for checking collisions with circles, so that's what I've been working on. I find it kind of sad that I've skipped over doing the low level graphics or audio systems by using XNA, but I'm still working on something that should be considered already solved.
Circles vs. Line Segment Collision:
If I add a little gravity, the circle slides nicely down the slope. And I'm handling the endpoints, so there's no gap, no matter which way you push the circle against the line. If anybody wants to see the source, just lemme know. It's not terribly complicated.
This stuff is all mostly still from the N tutorials. I just recently learned that a version of N is coming to xbox live arcade. Happy times.
On an unrelated note, I finished Bioshock last Sunday. (I didn't do much in my free time but play it once I got it.) Freaking fantastic. I'm looking forward to playing it again.