Either I have failed to notice something important, or I have a bug that 
needs to be explained/fixed.

I use passwd -l username (as root, of course)
It does what the man pages says it does.  Inserts a ! into the front 
character of the password in shadow. i.e. Actual entry from shadow:
jbc:!G3oMgW3hB93nI:11135:0:99999:7:-1:-1:134530724

The problem arises when I use passwd -u username

It removes the ! okay, but appears to scramble the password field.  i.e.

jbc:G3oMgW3hB93nI/bin/f:11135:0:99999:7:-1:-1:134530724

Obviously adding something looking like the beginning of /bin/false to the 
end of the password field making the original password unusable.  This 
addition is random depending on the user you lock/unlock.

Bug or not?
Red Hat 6.0 i386