I just started a similar kind of site (see signature) but what I'm doing is more personal, what you have planned is a lot more ambitious! I think it's a great idea, competition can be a really great motivator and a lot of people coming together like that could be a great way to brainstorm some really unique and worthwhile 30 day trials.
Also, you should take a look at the site
43 things, they do something similar and you could probably get an idea of what will work well from looking over what they're doing.