As has been mentioned, there's a monthly challenge thing going on in Flash Fiction forum, that could prove a source of writing prompts. (This runs every month from the 15th, 14 prompts will be posted over 14 days. Check out the September and August prompts threads, as examples.)
As well as the FF prompts, if anyone has any thematic or character- or plot-driven topics in mind, post 'em here. One can be chosen at random, or a vote can be taken so folks get kick-started by the same topic and we see the different kinds of stories we produce.
Additional ideas welcome, let it all out here.
-Derek (Western forum moderator... tho' he sometimes forgets this!)