What Ujjal says is essentialy correct.
The manufacturers who sat on the RDM working group did get a bit of a "leg up" on the competition....they saw , and directed the specification and were able to build products ahead of the actual release of the specification.
Also, and perhaps more important, as members of the working group these manufacturers (and individuals) were a part of the specification process and were in a better position to understand the subtleties of how RDM was supposed to work...ie all of the stuff that is not put in the specification.
As we all know there is a big difference between a standards specification and an implimentation guide.
Of course, "RDM Ready" is a bit of marketing ploy.
As one who has designed several products over the past few years that were marketed as "RDM Ready " and "ACN Ready"....
RDM Ready can be as simple as a product with a bi-directional 485 Tranciever on the front end an some extra space in the eprom or flash holding the code.
When we designed the Pathport in 1999/2000 we just jammed in as much spare Ram and Flash that we could economicaly at the time and called it "ACN Ready" even though at the time we had no clue as to how big the final code foot print for ACN would be.
This same principal held for the Sandnet boxes I designed, both the RDM and ACN "capable ??" devices were based upon a "best guess" at the time.
When we designed these devices we sat on the various working groups so we could see where the specification was heading but it was still a bit of a guess.