By the Nine Divines! I'm still just a spreadsheet initiate, but that auto formatting was steadily becoming my nemesis. I can hardly conceive of the headaches this has caused in people who make serious use of them. It's hard to believe it took this long to implement a feature that seems like it should have been there from the start.
I wouldn't get too excited, its yet another way Excel can mess up your data without realizing. Clippy never died, he just silently lives in Excel, messing with all the Clippy haters.