the DOCSIS system doesn't work like a phone modem or DSL, Cable modems "help" eachother, that is why they have 46mbit bandwidth.

Some cable modems (e.g. Motorola SB4000+) have "online" switch on them, that can help energy saving as I assume it turns off the NIC part but the cable modem is a device which is designed never to turn off.

Instead of turning off, I would disable unused ethernet ports (I got 2 on quad G5), replace old technology light bulbs with new tech ones, use a low CPU "real" screensaver (like computer name), set monitor off to 20-25 min (as this is CRT) and most importantly, on my machine, enable "Automatic" in CPU performance options.

For routers, I believe some does very effective energy saving, like sleep mode when no clients connected and so on.

I am one of the old fashioned guys who turns his computer off when not in use, if I won'T be around for 2-3 hours, I just make it sleep.