What percentage of the people in SO program in C#? What percentage program in Java?
Do these percentages change as a function of reputation? If so, how?
In general, since the Java and C# tags are the most active, as you go higher up the rep scale, you will find more and more Java/C# devs because it is easier to earn reputation in busier tags.
Well, for questions asked you could look at the numbers on the tags;
For the site as a resource for people with questions (i.e. who is going to read your question)... here's some page stats for per-tag searches that I've munged a little bit - I've scaled to 1 (so everything is relative to C#) since I don't think I should post raw page counts...:
I can't speak about how this compares to rep without a lot of analsis - but certainly the popular tags are very well represented on SO.