It turns out that it's something very easy to be done in Adobe Illustrator CS4. All you need to do is check the checkbox "Use Artboards" on the Export Dialog.
By the way, I am working on moving some artwork from Illustrator to PowerPoint, and found out that:
- Illustrator exports PNG as raster, which is not too bad.
- PowerPoint 2007 takes PNG pretty well.
- PowerPoint 2007 does NOT take Illustrator file directly.
- I hate the default way of inserting picture in document. It will automatically put it in the content box. So I have a habit of deleting the title box and content box on a default PowerPoint slide first, and then import the PNG. As long as the PNG document size ratio fit the PowerPoint Slide size ratio, PowerPoint will automatically fit the slide. I found this feature very helpful.