We've all seen fine custom cabinetry made to hold every television imaginable, well almost.
The question is, does anyone know who carries the most comprehensive quality retail selection of cabinetry designed specifically for 16:9 sets?

The snide answer is no one. The furniture industry lags years behind us. I think they've finally caught on to 35" tvs, fifteen years after their heyday.

multiple vendors

BDI
Sanus
Studiotech
Schroers and Schroers
just a few we use

I'm sorry are those widely available on a retail level? Can I get one at my local furniture store without a special order? Are the prices comparable to what you would find for the widely found 4:3 cabinets? Tell me more.

I take it from the tone of this post, that when you say "retail", what you really mean is... available at my local furniture store. If you want to look at some nice cabinets for audio / video, I suggest looking at your local audio / video retailer.

Some of the other links in this thread offer good choices. I personally like Salamander:

http://www.salamanderdesigns.com/

This is very nice quaility furniture. They offer a number of looks that you can customize to your liking. I don't believe most locations that carry it really stock it, they have showrooms set up with some typical pieces, and will order for you. My experience with Salamander, is that I've received most items in 2-3 weeks.

I'm in the same boat. Salamander seems to be the only one to offer the ability to hold equipment and also give you the option to add storage for dvd's, cd's, cables, etc. I did run across a west coast company that will build the furniture to fit the TV. They STARTED at $1000 per foot. Salamander isn't cheap either but it's a lot less than that.

Studiotech also has all kinds of media storage options.