John Gruber sums the change up sussinctly:

Twitter now identifies your @username anywhere in a tweet, not just at the beginning.

But my favorite from the actual post is the closing line:

Today’s update better reflects how folks are using Twitter now.

The @Reply feature and how it’s matured over time is exactly why I’m a fan of Twitter: they watch for emergence in how we all use it and tweak it appropriately. Evolution. And they benefit too.

[Via Daring Fireball]