JS / CSS ressource files and query strings for caching

Rails appends query strings to ressource files for intelligent browser
caching (only updating browser cache when files get channged)

But this article (
http://www.thinkvitamin.com/features/webapps/serving-javascript-fast )
the following:

At this point, you might ask why we don’t just add a query string to the
of the resource - /css/main.css?v=4. According the letter of the HTTP
caching specification, user agents should never cache URLs with query
strings. While Internet Explorer and Firefox ignore this, Opera and
don’t - to make sure all user agents can cache your resources, we need
keep query strings out of their URLs.

Any comments on this ?