Stars are, in fact, exportable. The "Google Takeout" zip archive for Reader includes a file called starred.json, containing a list of your starred items in a JSON format that, as far as I know, nothing else reads. If whatever reader you switch to can import marked items, you can probably work up a script to convert Google's format to whatever the new reader uses.
Thanks! The last time I checked was when they turned my google-account-with-an-apps-domain-email-address into an actual google apps account, and I had to move my Reader data.
