I don't think I've ever seen a keyboard with 'power-on' (scratch that, didn't that icube think have it, as well as some other macs?), but I don't think I could in good concience recommend one if I had. in my experience, keyboards with hotkeys for sleepmode, volume, and other extended functionality are slightly reliable at best.
you could rig a button, attached externally to the keybaord, that is connected to the power swith jumpers on you mobo. it would look funky, it would necesitate some minor case modding (just drilling a hole really), and it would be a painus in the anus whenever you need to unplug your keyboards, but it would work.
the real question, though, is why is you computer out of arms reach?