Personally, I would only say "someone" in this context if I hear a voice or some movement that lets me know that there is indeed someone who wants to make a statement. So, we're already expecting the answer to be "yes". If you use "anyone", you aren't sure what the answer will be.
Likewise, in PrinceGuillermo's example, if I'm in a group that is supposed to be just 10 people, and I count 11 heads, I might ask: "Has someone else decided to come with us?" even though it is a question, because I'm pretty sure the answer is yes, and I'm just trying to figure out who this person is.
So I think both PrinceGuillermo and Reina are right, but at the same time it's not always as simple as "question vs. statement" or "general vs. specific". Anyway, that's how I used to explain it to my advanced students. Does that make any sense? At any rate, this subtle nuance is often lost in everyday language.