SFScope has a post about a new short story contest sponsored by Hadley Rille Books and the National Space Society (NSS) called "Return to Luna." They want science fiction stories that capture the pioneering spirit of a moon base. Stories should be set in the near future 50-150 years. Check their submission guidelines for more details.
Entry deadline: June 15, 2008
Length: 2000-6000 words
Prize: Winning stories will be published in an anthology
And remember this great advice about entering writing contests from the Novel and Short Story Writer's Market blog.