I saw a KA-BAR D2 Combat Knife #1464 today at SMKW's rep day that had the best tactical sheath I've ever seen. It's made by Eagle, and is a top quality ballistic nylon over a hard casing for the blade (I don't think it's Kydex, but it's rigidity was similar), and the inner surface was lined with felt. Bump the sheath and it's quiet, unlike Kydex. Inserting and extracting the blade is totally silent, again, unlike Kydex. Yet it has the safety of Kydex-like rigidity. It doesn't snap into place like most Kydex sheaths, but it has a tight fitting strap on the grip that secures it very well.
Seems to me that this avoids the dangers that A.G. points out, without giving up the quiet nature of leather. It's not perfect, but it's the closest thing I've seen.
I don't know if this design is available for other knives, but it needs to be. If my life relied on a safe, yet quiet, tactical sheath, I'd pick this one over anything else I've seen.
Post #500 - Yiipppppeeeeeeeeeeeee!!!!!!!!!!!!!