Saving in native Illustrator format There is a known issue in Illustrator CS where the native Illustrator 10 format is not shown as an available option. Selecting the option "text" will save in the proper format. MacFixIt reader Charles Reeves writes:
"After I installed Illustrator CS on my G5, the Save as... menu in Adobe Illustrator 10 doesn't look like it will allow you to save the file as an AI file anymore. Instead of Illustrator being listed, it shows "Text" and the Illustrator format doesn't even seem to be an option. "
Again, selecting to save as "Text" will actually save in native Illustrator 10 format.
On a related note, there are also some reported problems saving PDF files in Illustrator 10 where text appears jagged when opened with Adobe Reader. Turning on "smooth line art" in Reader's preferences seems to eliminate the problem in most cases.