Robert Pearson has the 2012 edition of the Chess Carnival off to a big start with The Best Of! Chess Blogging Part I: Openings. Logical place to begin, right?
One of my own posts is included and openings study has been a recurrent theme of this blog, despite the common advice - which one can find repeated in various places, sometimes with great solemnity - that improving players should not study openings until reaching a relatively high rating threshold (take your pick, anywhere from 1800 to 2400). Openings study is termed a waste of time for lower-rated players and the argument is made that such study will not materially affect their results. (I'll note that sometimes similar advice is given about studying endgames, but that's another topic.) In any case, this advice strikes me as somewhat facile; I wonder if a majority of players giving such advice actually followed it themselves while moving up the ratings ladder.
I do acknowledge the validity of the basic argument, in my experience most clearly articulated by Dan Heisman (for example here), which is that until a certain threshold of playing strength is reached, players lose games mostly because of poor tactical play. Therefore, a player of up to 1400-1600 strength will get a much bigger return on their investment by concentrating on studying tactics and improving their thinking process. This is mathematically demonstrable, not just opinion, so I respect that.
A more general argument is that "opening theory" is useless (see Dana Mackenzie's provocative post referenced in the Carnival) and/or so is memorizing opening lines. Unlike the previous argument, this one is not so clearly provable as a theoretical statement. In practice, however, the point is taken that mindless memorization of move sequences or attempting to keep up with all the 20th-move theory improvements of professional players are not really worth doing. This probably should be common sense, but it seems enough players do these things to generate the "don't study openings" advice. (I confess I began my tournament career as a Class C player memorizing lines from Modern Chess Openings, a practice which I've since broken to good effect.)
This all still leaves the question of when an improving player should begin "serious study" of openings. For example, NM Heisman in the article linked to above noted that he hadn't "learned a great deal about specific opening lines" until he was a Class A player (around 1900), but that's a somewhat fuzzy description of the state of his opening knowledge and study practices at the time. There is also the related question - to my mind, the most important one - namely, what exactly is serious openings study?
The Opening Study Methods post addresses this in more detail (with some practical examples), but the key for me is having openings study fully integrated into one's overall approach to the game. This means achieving a deeper understanding of your chosen opening and how it affects the entire game. This includes a recognition of common early traps, standard middlegame plans, and long-term factors such as decisions affecting pawn structure that can carry through until the endgame. Someone who is a true openings expert will be able to explain both the reasons behind each move and its consequences. (Substitute the word "chess" for "openings" in the above statement and you may find that it's even more to the point.)
One could argue that a deeper level of understanding of the game is beyond the comprehension of (____) rated players (fill in the blank as desired). However, chess improvement (or any skill improvement) is the result of "effortful study" in which a person continually tackles challenges just beyond their current competence (pushing their envelope, in other words). I would argue that any meaningful effort in this direction is worthwhile when it results in greater insight into the game that is practically applicable. Take a look at this recent post at the Prodigal Pawn, for example.
As an amateur chess player, I am under no illusion that my own study practices are comparable to those of professionals, nor are they even necessarily optimal for someone at the Class level. However, I have to say they're working pretty well in practical terms for me and I've had similar positive results in the past when I've undertaken serious openings preparation (see Annotated Game #1). Openings study has been a major contributor to solidifying the foundations of my game and a good method for overall improvement, when pursued in-depth.