Oh wow, we really needed a third ortho thread to keep track of.
As for felts, what I think makes it good for our purposes (and here I may repeat something wualta has already said) is that it has incredible density, while still allowing air to pass. And in my limited experience (about 20-30 different types of fabrics/felts) I'm pretty convinced that natural fibers are better at absorbing energy than shiny synthetic fibers, which might be why kabeer's fake silk does nothing much to bass.
One other thing to note is that placement has as much to do with the damping results as the kind of material you are using. We basically have three "damping zones", each one with different effects: the driver itself; the vents (if any); the inside surface of the cups (and baffle, maybe). While the first two are similar in that they trap air pressure, and just offer different effectiveness (the nearer the felt to the driver, the larger its effect), damping the inside surface of the cups just kills reflections.
Two different things, which sometimes might be combined as in the "standard" bass heavy HP-3 mod which uses a very thick disc of felt (smeggy's felt) to both damp the back of the earcups and the vents.
A second, finer kind of placement is where you apply damping on the drivers: we found out with kabeer that by varying the damping density on different areas of the driver, you can roughly select which frequencies you damp. So by making donuts, or a combination of donuts and discs, you can for example raise the treble without killing off too much deep bass, using felt instead of a reflex disc/dot.
My experience with the Yamaha I have/had and with the T20v2 is that usually you need a dense, thin layer pressed against the driver (kept in place with foam in the HP50, smeggy's felt acting also as a spring in the hp3, the driver assembly in the t20v2), and another, usually lighter layer (or two) for the vents and cups. Then it's a matter of finding a balance between vent damping and driver damping.