How can I determine whether my system if affected by this issue? If so, can I work around it, and also determine which users’ passwords have been exposed?
As stated in the article linked in the question:
Anyone who used FileVault encryption on their Mac prior to Lion, upgraded to Lion, but kept the folders encrypted using the legacy version of FileVault is vulnerable. FileVault 2 (whole disk encryption) is unaffected.
The password shows up in
/var/log/secure.log. To look for it, log in as an Administrator, open Terminal.app and run
sudo grep -i passwordAsUTF8String /var/log/secure.log
Have a look at the result (if any) to see whether passwords appear in plain text.
For additional information see