Shin Okada said:
Then my answer is, definition of "official" may be different in each play group.
Rule book is official
Errata is official
FAQ is official
Sage advice is official
Customer service answer is official.
If you use the strict definition of "official", that means something WotC does say official, all the above documents and answers are official. Made under the name or approval of WotC.
But IMHO, credibility of each sources are quite different. And whether if a DM (or a play group) regards some or all of them as "official rule change" is a quite subjective choice.
Fair enough. I'd say "offical" serves three purposes:
1. What rules are used for tournament play.
2. Base line to draw from in deciding your own rules for your game(s).
3. Predictability and consistency from game-to-game so you can play with more than one group and have a consistent set of base-line rules.
With that in mind, it is importatn to distinguish between what is an "offical" rule and waht is not.
FAQ entries are "official, certainly, but, more importnatly, they are allowed by WotC to change core rules. General Sage advice and customer service are not allowed to do that.
Thus, I think the published rules are the books plus errata documents plus FAQ, insofar as it is used to publish additional errata only.
The trick, of course, is to know when the FAQ is being used to publish additional errata and when it is not. The difficulty of doing this leads to a very reasonable decision by some folks to disregard the FAQ for that purpose, and I can understand that.
Still, I think that I have shown that WotC does indeed use the FAQ to publish additional errata that is not in teh errata documents.