Mr. Corbett notes that the word appeared in 18 different articles in the last month across a range of sections, and none of these articles (we're guessing) were about ovenbirds or black-capped chickadees.

From Mr. Corbett's memo:

One test is to ask yourself whether people outside of a target group regularly employ the terms in question. Many people use Twitter, but many don’t; my guess is that few in the latter group routinely refer to “tweets” or “tweeting.” Someday, “tweet” may be as common as “e-mail.” Or another service may elbow Twitter aside next year, and “tweet” may fade into oblivion. (Of course, it doesn’t help that the word itself seems so inherently silly.)

The Times doesn't do silly.