Supposedly this was how it was supposed to have been working all along, and they FIXED what they considered a bug and not introduced one.

As Souca stated, this doesn't completely negate the possibility of a man-in-the-middle attack, but it reduces the window of opportunity from 30 seconds down to how ever long it takes for you to press enter after typing the last digit of the code. More security for 99% of authenticators vs 2 minutes of hassle for 5-boxers... it's not too hard to see why Blizzard made the choice they did in fixing the bug.