Absolutely. You should be developing a marketing plan at the same time you're developing the app. Figure out the best way to generate some positive buzz, be it through blog updates, screenshots showing your progress, etc.
Don't go overboard, but start getting people excited! And while you won't be executing a good portion of your marketing plan until the game is released, you need to know what that plan is.
Thats right, the least we can do is figure out, what to do once its out. A development-blog can't hurt either I guess. Perhaps for some people it's interesting "how games are made", perhaps targeting a more non-techie crowd.
Don't go overboard, but start getting people excited! And while you won't be executing a good portion of your marketing plan until the game is released, you need to know what that plan is.