Although this news is slightly dated (a bit over a week now), I only heard about it yesterday. Motor City Online has been axed effective the end of August.
For those unfamiliar, the game is a Massively Multiplayer Online(MMO) car racing game, where players race to win money, to buy parts to tune up their cars, to race to win money...
According to the [development team page, the reasons for shutting down the game (now running a bit more than a year) are mainly related to shelf space and the inability to keep the boxes that the game is sold in out in the public eye.
After talking to folks here at the GDC, that doesn't seem to be the buzz inside of the industry. Clearly, and online service that is doing reasonably well with a stable customer base doesn't necessarilly need shelf space to keep the existing patron's playing. That's the joy of the customer base being "stable".
Further, on a game with sufficient word of mouth and lack of interest from the retailers, the company can sell new customers online. The problem comes when the customers start to flee the game, and aparently this was one of the problems with MCO.
According to developers (at other companies), part of the problem is the peer- to-peer nature of the MCO client. Due to the way some of the communications happen during the game, it is possible to grab the IP address of another player and mount a Denial Of Service (DOS) attack on their client in order to slow down their reaction time and cause them to lose. Given that this is a skill-based game that requires timing, this is a major issue for fairness and gameplay.