As we posted yesterday, Numbers ‘09 uses a different file format than Numbers ‘08
You can save Numbers ‘09 documents in a format compatible with Numbers ‘08, the option is just a little hidden. 
Here’s how to make your Numbers documents compatible with the older version: First, choose File > Save as… Then, click the checkbox at the bottom labeled “Save copy as:” (the default is Excel Document). Finally, click the drop down menu (as shown below) and choose iWork ‘08.
The new file will have the same extension (.numbers) as the ‘09 file, but it will be saved as a “package” file, rather than a “flat” file.
For more explanation of the new file format, see yesterday’s post: File Compatibility Between Numbers ‘08 and Numbers ‘09.
