Apple officially announced the lineup of iOS devices that would be compatible with Game Center; which has a surprising team player on the squad. Although the iPhone 3G's exclusion might be seen as a shock for some people, Apple's decision to keep the second-generation iPod touch might be regarded as an even bigger shock. Regardless, those sporting the iPhone 3G, even the original iPhone, will have to live with the fact that they're not going to experience all of the neat things expected out of Game Center.
Although the second-generation iPod touch and iPhone 3G were released around the same time period, the iPod touch packs a faster 532MHz processor versus the iPhone 3G's 412MHz one – so maybe that could've been its downfall.
Apple iPhone 3G Specifications | Review