Whatever you want to call them, the idea of having a theme for everyone's submission on a particular day has been floating around since the project's inception. I think Cindy brought it up first and we decided to first determine how successful this project could be without those parameters. Speaking for myself, this has been great fun and I think that its been more successful than I hoped for. Ryan has brought up the topic of themes again in a comment earlier this month and I think we can discuss it more informedly now that we have some members with almost 50 posts.
Questions I have are: Do you want themes at all? Would you like them to be integrated with your One A Day project or something separate, as Ryan seemed to be suggesting? Themes for a specific day? Or would you like a weekly theme where you'd try to get your photo sometime during the week and relieve the potential stress of trying to find a specific shot in one day?
The most fun question is about what the themes might be. I would like to recommend a wonderful magazine called Cabinet as an example of some great assignments. In the begininning of every issue they have these wonderful commissioned articles about colors, objects, foods, etc. I think their views of photographic assignments would tend towards more obtuse themes, not necessarily involving specific objects.