They will not bring clan lists back for Halo 3. There are several reasons why, IF they were to add a clan list, they would do it via the dashboard:
As a company, Microsoft is looking to sell games, not consoles. Yes, the console is the first part of this equation, but (especially after filling warranty issues... heh) games are going to be where they continue to rake in the money. By providing a clan list in Halo 3, they would be encouraging people to stay on that game and not expand their gaming experience. They are also avoiding major cheating and glitching issues that they had in Halo 2, by bringing out new games - gamers move on to other games, and rank on Halo 3 is not the all-important status that it was over in Halo 2. This encourages GAMERSCORE pursuit... bringing us back to them wanting to continue to bring in revenue by selling more games.
Now, back when everyone was still crying for a clan list, a friend of mine had called XBox Live about issues that they were having. One of the things that they asked the person was whether or not clan lists would be brought back, and this helpdesk person said that they would be doing something about the clan lists.
This is the "Silver Tag" clan list as it stands now. They have provided people who really want some sort of a clan with the ability to organize clans. Major Nelson stated how to use this tag as a clan list. This is their solution to quiet people. They have provided what they are going to provide - cost of development and servers just to host clans on the dashboard is not going to make sense for them. They don't make money from clans (or so they think), and those of us who really want clans, can organize without them. They kill two birds with one stone --people requesting larger friends lists by creating the ability for them to look at other people's friends lists, and people crying for clan lists in one swift and relatively easy development move.
On the note of repeatedly requesting things in the same thread - we would do the same if you were becoming a nuisance on our forums as well.