A number of users have reported an issue in which subjects are missing from messages in Mail.app after the update to Mac OS X 10.6.4.
This problem can generally be fixed via the following process:
- Launch Terminal.app (go to Spotlight in the upper-right corner of the screen and type “Terminal”)
- At the command prompt, type: defaults delete com.apple.mail TableColumns then press return.
- Quit and relaunch Mail.app