Ethernet appears to be included.
http://www.apple.com/imac/
"Of course the iMac G5 offers all the right ports to connect to your universe with ease. Starting with the new headphone jack that’s also a mini-optical plug. So you can watch DVDs and listen to them in 5.1 surround sound. You’ll also find a passel of USB 2.0 and FireWire 400 connectors for your camera, camcorder or gamepad. Or if you want to connect your iMac to your TV or a digital projector, the mini-VGA port gives you the option. The line in jack lets you record an electric guitar into GarageBand. iMac offers Ethernet for wired networking as well as a modem for dial-up Internet. Your modem can also double as a fax machine."
http://docs.info.apple.com/article.html?artnum=86813
"The iMac G5 has a built-in 10/100 Mbit/s Ethernet port. The iMac G5 computer can be connected to an Ethernet cable from a cable or DSL modem, hub, switch, or router, or to another Macintosh computer. The connected device can be either a 10Base-T or a 100Base-TX device; the port automatically detects which type of device is connected. You don't have to use an Ethernet crossover cable to connect to other Ethernet devices."