And by "Killer"... I mean, of course, terrible.

Assuming Apple advances Game Center (GC) to directly compete with Microsoft's Xbox Live (XBL) system, some interesting apps will develop out of that ecosystem. Here is an idea for an app that is not only a total dick move, but would also probably get rejected in a heartbeat. I'm calling it now, though. Someone's going to try.
First, let us examine what sets GC apart from XBL and why it is not a direct carbon copy. One significant difference is that XBL is highly score focused with online play with friends often being a competitive forum. GC appears to be a more casual environment, with friends scores being listed for "All-Time", "This Week", and "Today". To me, this is designed more to update a user on what his/her friends are playing and bring in a low-pressure, fun-focused level of competitive play in which a user can reach the Daily Leaderboard, without having to worry about beating the All-Time score.
Microsoft has done amazing things with XBL and set the standard for what an online gaming community should feel like. One way they've managed to do that is by setting controls on their developers. All Game Upgrades must be purchased with XBL Points (Apple is set up to compete via In-App Purchase) and developers are restricted concerning Achievement Points. Typically, a feature title has 1,000 XBL Achievement Points to unlock, with DLC usually clocking in at around 250. XBL Arcade games range in point assignment, but most are under 300 total. GC Achievements, by contrast don't even show up in a user's profile, and a user's total score can only be calculated manually by totaling up all the individual achievements earned. Games have a budget of 1,000 GC Points, and no single Achievement can be worth more than 100 points.
However... this will all change as GC evolves, and certainly if they open up an API that allows a developer to create apps that promote a user's score and achievement tracking. I'm expecting this to happen within a year, especially as GC moves back to the Mac in Lion (and hopefully the Apple TV someday soon). With this accesiblity and userbase will come real competetive players.
So, since app publishing is so simple, and GC integration is so easy, I'm sure developers out there will make games for $0.99 (or ad supported) that allow a user to boost their score by 1,000 with some easy (like... click this button, easy) achievements... and the crazy thing is, an app like that would sell like crazy if GC scores were as public as XBL scores are. I know people who will spend weeks working away on a game they don't even enjoy anymore simply to unlock all the achievements, and as we all know, cheating for scores totally counts.
Just like the "Flashlight" apps that quickly become known for doing other secret things like tethering, it would be easy for these "Jackpot" apps to get known and talked about, and I think they'd actually do really well. The missing piece of the puzzle is getting them approved by Apple, so they'd actually have to provide some real value to the user other than just boosting scores. I can't wait for the tantrums.
### [[EDIT]] --- This will also get interesting as games for Windows Phone 7 also can contribute to a user's Xbox Gamerscore. I'm less familiar with app publishing for Microsoft's platform, but I imagine since they're hungry to get apps in, they're pretty lax right now on what constitutes "unique and valuable".