I switched to long headers in the view options. I now want to revert back to the default view. However, going into View -> Message -> Default Headers doesn't help.
Hmm, works for me. Have you tried re-launching Mail afterwards?
Do you have a lot of custom rules / mailboxes set up? If you dont mind wiping your preferences then you could try deleting the Mail preference list.
You should also be able to reinstall Mail from your system Disks. Try using Pacifist so you don't need to do a full reinstall.
I switched to long headers in the view options. I now want to revert back to the default view. However, going into View -> Message -> Default Headers doesn't help.