Please don't get to work scaling your architecture yet. Work on making your app awesome and wanted by lots of people.

Trust me, I've been bitten by this problem myself. It's so easy to get really worried about the number of hits you're going to take. It's also so easy to get really excited about the scaling challenges.

But, quite frankly, the chances of you getting featured on yahoo.com are miniscule! First focus on getting there by creating something cool. Then focus on scaling.