On a Lion machine first try rebooting (seriously)
It's a known issue.
If that doesn't work:
Use Go > Go to Folder to go to "/private"
Select "etc"
Command-I to bring up the Information pop-up (don't close it again till the end of this procedure)
Click the lock symbol in lower right corner to unlock permissions
Make a note of the permission setting for "Everyone"
Click the permission setting for "Everyone" and select "Read & Write"
Use Go > Go to Folder to go to "/private/etc"
I found that "shells" was empty, but there was a "shells~orig" with a good list of shells in it
Delete "shells"
Select "shells~orig"
Command-C, Command-V to copy-and-paste it; this creates "shells~orig copy"
Select "shells~orig copy
Click again to enter rename mode and change name of "shells~orig copy" to "shells"
Return to the Information pop-up and change the permission setting for "Everyone" back to its original value
Close the Information pop-up
