SciArt in America is running its first flash fiction writing contest, and entries are being judged by none other than Jeff Vandermeer:
The theme of this flash fiction is ecology, defined as the study of the relationship between organisms and their habitats. The winning story will be selected based on how well and how creatively it explores this theme.
Entries are a max of 750 words. I can't find any payment information at this time unfortunately.
Check it out here: http://www.sciartinamerica.com/uploads/6/0/8/9/6089526/flash_fiction.pdf