ALOTT5MA FRIDAY GRAMMAR RODEO: The New Yorker's Mary Norris
explains their whole thing about the diaeresis:
The fact is that, absent the two dots, most people would not trip over the “coop” in “cooperate” or the “reel” in “reelect” (though they might pronounce the “zoo” in “zoological,” a potential application of the diaeresis that we get no credit for resisting). And yet we use the diaeresis for the same reason that we use the hyphen: to keep the cow out of co-workers.
Basically, we have three options for these kinds of words: “cooperate,” “co-operate,” and “coöperate.” Back when the magazine was just getting started, someone decided that the first misread and the second was ridiculous, and adopted the diaeresis as the most elegant solution with the broadest application. The diaeresis is the single thing that readers of the letter-writing variety complain about most.
There's a poll on the right-hand side, for old time's sake.
