Believe it or not, the Perspective Grid tool in Illustrator CS5 isn't exactly new.
The tool, or at least a less-capable iteration of it, featured in Macromedia's FreeHand 9. Of course, Adobe went on to buy Macromedia and has finally ported it to Illustrator. There's no doubting though, the Perspective Grid is a very handy tool to have at your disposal - enabling you to draw in perfect perspective by automatically mapping vector artwork to a predefined grid.