Apple just seeded a new build of iOS, version 4.3, to developers, and it looks like they finally dropped support for the 2008 iPhone 3G. The device is indeed not listed on the iOS developer website, and nothing makes us think that future beta versions will be released for that model. The latest OS for the iPhone 3G may be iOS 4.2.
AT&T discontinued the iPhone 3G last June. Several users who installed iOS 4 on the iPhone 3G last year lamented that the OS was slow and unusable, mainly because of the slow processor and less RAM on the 2008 device. Apple tried to make things better with iOS 4.1 and 4.2, but the iPhone 3G was clearly showing signs of its age.
Update: there's iOS 4.3 for iPod touch 2nd generation either.