Quicklook knows how to read pdf, so as long as you save the Ai file with "Create PDF Compatible File" checked, Quicklook can display it.
Yes, but that creates larger files and I have a lot of older AI files that don't have pdf previews...
So, CS5 doesn't add generic Quicklook compatibility?
One of the many reasons I save all my Illustrator files for production out as PDF's with Illustrator editing capability is because the quicklook is so convenient. Even though it is a larger file then just the .ai file it is still smaller then an .eps.