iPhone 3Gs. Message: This device isn't eligible for the requested build.

Try updating iTunes and disabling your security software/firewall. From http://support.apple.com/kb/TS3694:

This device is not eligible for the requested build (Also sometimes displayed as an "error 3194")

1. Update to the latest version of iTunes.
2. Third-party security software or router security settings can also cause this issue. To resolve this, followTroubleshooting security software issues.
3. Downgrading to a previous version of iOS is not supported. If you have installed software to performunauthorized modifications to your iOS device, that software may have redirected connections to the update server (gs.apple.com) within the Hosts file. Uninstall the unauthorized modification software from the computer.
4. Edit out the "gs.apple.com" redirect from your hosts file, and then restart the computer for the host file changes to take affect. For steps to edit the Hosts file and allow iTunes to communicate with the update server, see iTunes: Troubleshooting iTunes Store on your computer, iPhone, iPad, or iPod—follow steps under the heading Blocked by configuration (Mac OS X / Windows) > Rebuild network information > The hosts file may also be blocking the iTunes Store. If you do not uninstall the unauthorized modification software prior to editing the hosts file, that software may automatically modify the hosts file again on restart.
5. Avoid using an older or modified .ipsw file. Try moving the current .ipsw file (see Advanced Steps > Rename, move, or delete the iOS software file (.ipsw) below for file locations), or try restoring in a new user to ensure that iTunes downloads a new .ipsw.