I have to say, now that I know what happened this will never really bug me again, but sweet Jahosafat it was annoying for a long time until I asked the list.  I think that that this could be handled like set-goal-column or downcase-region or upcase-region or narrow-to-region all of which need to be manually disabled once.  They all fit the same mold.  Some really useful and seemingly innocuous feature that could confuse the smack out of some poor unsuspecting user.  
 
John
 

________________________________

From: Drew Adams [mailto:drew.adams@oracle.com]
Sent: Mon 2005-11-21 18:27
To: emacs-devel@gnu.org
Cc: John Russell (jorussel)
Subject: RE: FW: Emacs non-ascii characters



    Perhaps if the current locale is en_US (and other en variants, since
    many use the US layout and have little need for input methods),
    then C-\
    should not automatically select an input method the first time it is
    hit. Then the user will be prompted and can hit C-g. This will overcome
    the problem that if it is hit by accident by someone who knows nothing
    about input methods, they have no way of knowing what has gone wrong.

That sounds like a good approach, provided the initial prompt were only
asked the first time (and the setting remembered persistently, via
Customize).

But again, I can't speak for people who use this. Would en_US users who use
`C-\' be bothered by an initial prompt?