I have the iPhone 3G somewhere in the basement. For years, it has been a paperweight, it couldn't download anything from the App Store, including apps that originally were compatible. It even lost some capabilities it originally had out of the box, for example it could not sync contacts with Google anymore.
The last compatible download feature doesn’t work on my 1st gen iPod touch running iOS 3, but it does work on my iPad running iOS 5. I’m not sure about iOS 4.