I think it does!
I've been thinking about this topic as I've prepared a proposal for the IRA (International Reading Association) 2008 conference. My main idea is that there are many crossover skills between fiction and nonfiction such as focus, organization, including appropriate details, and practicing writing mechanics. On top of that, writing a short story that the student has brainstormed herself is more fun (and actually more work, but don't tell anyone!) than writing the traditional 5-paragraph essay. I also think that students who have practiced using figurative language, imagery, and literary techniques when writing a story will be more likely to use these devices when writing nonfiction. With the addition of the written portion of the SAT, it seems that fiction writing can be one way to help students become more well-rounded writers comfortable using a variety of literary devices.
If you have had experience teaching fiction to intermediate or middle school students and have seen crossover of these writing skills to essays or research writing, please e-mail me at firstname.lastname@example.org.