Last night I decided to increase the scale of the planet to Earth's size. The original size of the radius was 60,000 meters. Earth's radius is about 6.5 million meters. So, I had to make my planet over 100 times larger.
Unfortunately, all sorts of problems arose from attempting to do this. First, my depth buffer went to crap. If I wanted to be able to see anything from even a low altitude, I had to extend the far clipping plane back to at least 1 million meters. This resulted in all sorts of errors with triangles that were fighting over their depths because the range was trying to be stretched over such a long distance.
The second problem that occurred was that my camera was rotating in "steps". If I moved the mouse up, instead of smoothly rotating the view upwards, it would snap every Pi/4 or so. This snapping problem was alleviated by shrinking the size of the planet. Shrunk in half it was still too bad to use, shrunk by 4x was much better, but the problem was completely gone if I shrunk by 10x.
I really need to fix all of these problems so that I can have Earth-sized planets. There are many other planets that are even larger than Earth so this is very important. I have found some articles online where people have found solutions to these problems. I'm just unsure of how feasible these solutions are for XNA (they are all OpenGL solutions).
On a side note, I switched my Perlin noise function from fBm to ridged multifractal and it looks much better. I am very impressed with the terrain results. I would post screenshots, but with the depth issues and such, I figured I would wait.