Plague Inc. is one of the most popular strategy games on the App Store: It's a title with the gory premise of you controlling a plague running around the world, trying to infect as many people as possible before the rest of humanity can come up with a cure. The game's latest update is out this week, and it adds a new virus to play with called the "Necroa" virus. This virus reanimates dead people and causes them to want to attack the living -- in other words, it creates zombies, and the update comes with an extra opposition force called Z Com to mix up the strategy a little bit.
But even more interesting than the new update to the game is how it's being distributed. If you can beat (or have beaten) Plague Inc. on the highest difficulty, you get the new content for free. If you can't, then you can get the content via an in-app purchase of US$1.99. I haven't seen an app do that before. Of course, a lot of apps have sold convenience items via in-app purchase, or used purchased items to let players reach goals faster, but I haven't ever seen a developer make an item available based on difficulty.
We'll have to see exactly how that works for them as the update goes forward. In the meantime, the update is live right now, and you can download Plague Inc., if you haven't yet, for 99 cents.