"Text is read-only"... except it isn't... or shouldn't be

[ken]

A file which I've been opening in emacs and editing for a few years in 
emacs suddenly with emacs version 22.1.1 became read-only.  I open it, 
can navigate around in it (though the cursor/mark isn't visible), but 
when I try to edit it, I'm not allowed.  I get a message in the 
minibuffer saying, "Text is read-only".  The beginning of the modeline 
is "-u: --".  I can open this same file in vi and edit it (as the same 
user) without any problem at all.  I don't have this problem with any 
other file I edit in emacs, just this one.  The OS is linux, kernel 
version 2.6.18.384, and the permissions for this file are 664 and I am 
the owner of the file.  If I change (i.e., mv) the name of the file to 
something else, emacs behaves normally and lets me edit the file just 
like any other file.  So I'm pretty sure that the problem is that emacs 
has a problem with the file name and thinks I shouldn't be allowed to 
edit it.  How do I turn this behavior off?

thanks much.