It's curious that this sort of game design is exactly the sort that's getting railed against in the progressive game design community. Fellows like Jonathan Blow and Jason Rohrer argue quite convincingly that games' focusing on artificial rewards are merely masking the lack of meaning that modern games have to offer.
The face that this solution works as well as it does suggests that there's not much meaning to the email overload we see today. That means there might exist a better solution to this problem, something that encourages meaningful interaction in the place of overloading responses. But this is an interesting stopgap solution in a lot of ways.
It makes sense. For some evolutionary reason our brains find many peculiar tasks to be very satisfying and in order to "find the fun" games are designed to tap into those. For example there are a huge amount of "organizing" games, such as Tetris, that play on the fact that for some reason people love to assemble things to be perfect and just in the right place. The games industry has been like a big research experiment to find these obscure things that for some reason people love to do. Now we can use this knowledge to improve existing products' usability.
The article already touched on the Nike iPod software, but another similar example of using a game for self improvement is the RPG bank toy that exists in Japan. http://www.wired.com/gamelife/2008/06/bankquest-bring/ With this bank/toy the money you put in also exists in a virtual world and so you can buy new armour for your avatar.
The Sixty One http://www.thesixtyone.com/ steals quite obviously from some of the recent developments in games as it will mimic Xbox Live and give you achievements for doing various music rating tasks on the site.
Myself I've considered a few ideas to use games in my work. When I was writing a tool to automatically compile help files for our code I considered trying to build in some sort of leaderboard that would show during a build who had commented the most in the hope that it would encourage people to comment every one of their functions. However there were too many technical issues to iron out at the time and so I didn't proceed with it. I think the overall concept though was a smart one (unless people started doing useless comments...)
"A Theory of Fun" convincingly argues that the fun in games is all about learning. Of course it is tilted towards learning stuff that was useful a few thousands years ago.
It's a no-brainer that games can change behavior -- they are disguised variable reward skinner boxes! I suspect that a part of being a great teacher involves implementing, as much as possible, an optimized variable-schedule of reward for each student.
I also suspect that a hobbyist or student who "gets into it" is often one who has stumbled into the right variable-reward situation.
This was one of the big motivations behind the design of StackOverflow. Joel and Jeff talked about it (a lot) in the podcast. Their badges were modeled very closely on the badge system of XBox Live.
Our Nitro platform provides game mechanics as a service that can easily be integrated into sites/apps/etc. to incent and motivate user behavior. You can check out the marketing speak here: http://www.bunchball.com and the actual meat here: http://wiki.bunchball.com
The face that this solution works as well as it does suggests that there's not much meaning to the email overload we see today. That means there might exist a better solution to this problem, something that encourages meaningful interaction in the place of overloading responses. But this is an interesting stopgap solution in a lot of ways.