There are many things beside auto-sleep that can save you power, in fact an LCD monitor consumes as much power as an idling Mac Mini (roughly 20W each). Simply setting your monitor to auto-off when not in use can save you vast amounts of energy, your typical LCD monitor consumes no energy while turned off by the computer. A CRT (for those who still use them) save phenominally when turned off, dropping from ~70W-80W down to ~2W of consumption, essentially meaning that leaving a CRT turned off but connected to power is less harmful to the environment than working an extra hour at it. These are all energy-star monitor figures too. Screen size and backlight also varies energy consumption, some 20"+ LCD monitors can easily hit 100W, while Dell's current line hit only 20W.

My core duo laptop with a 17" screen only consumes a max of 90W with a CCFL, a Mac Mini consumes up to 110W with an additional 20W for a display. So I'm still not sold on Mac Mini's as environmentally friendly desktops, as there are better solutions out there, namely my old laptop still beats the Mini in efficiency terms.

I'd also add that anyone using SSD should have a very short time (maybe 10 minutes) between sleep activating and hibernate activating. The fast start up times allowed by an SSD can maximize power savings while minimizing inconvenience, which is arguably going to be the best way to aid the fight against global warming.

Can you wake it up with an IR remote? If not, then auto-sleep is a non-starter for a set-top box. Just for example.