Also, Miguel, what's the difference between, say, a Cyclist's Rain Cap and the ID Sil Cape?
Erm... not much really. I just got the ID Sil Cape yesterday and have been comparing them. One thing about most cycling rain capes is that they are designed to be used in the wind (as much as it is possible to use a sail in the wind). They have wrist or finger straps to hold the front portion of the cape steady in the onrushing wind, and usually a strap inside the cape that you tie around your waste to hold the back of the cape from flapping around. Of course there is also the bicycle handlebar and saddle to help stabilize the cape, which walkers don't have.
The strap in the rear hem on the Sil Cape is great, but doesn't seem to work quite as well when you aren't wearing a small pack. But I haven't tried it yet, so I can't say much about it.
When I walked the Tour du Mont Blanc last summer quite a few ultramarathoners running in the North Face TMB Ultramarathon used bicycle capes to stay dry.