In case you haven’t noticed, files saved under Numbers ‘09 will not open under Numbers ‘08. In fact, the file format incompatibility issue also affects Pages and Keynote.
Why did Apple do this? Tom Reestman gives a good explanation over on the Apple Blog. The short answer: it makes files easier to share across systems via email.
iWork ‘08 files were saved as packages, iWork ‘09 files are saved as flat files.
