Historically OED updates have been released in sequential alphabetical blocks. The December 2007 update, for instance, ran from purpress to quit shilling. The March 2008 update operates on a different model. Rather than a alphabetical block, it consists of words with “significant lexical productivity” and words which will “benefit from immediate review within the dictionary.” In other words, it’s based on relevance, rather than alphabetical order.
Future updates, according to the OED, will alternate between the old and the new model, with the June 2008 update continuing the alphabetical revision from quits, and the September 2008 update switching back to relevance.
This strikes me an eminently sensible. It allow the OED to be (somewhat) timely, while also continuing the systematic alphabetical review of the entire dictionary. Way to go, OED.