Despite changes that should fix Opera issues, 2.36 doesn't work correctly for me in Opera.
I tested 9.27, 9.52, 9.64 and 10.00 under Ubuntu, upload is always interrupted early.
When searching for Opera iframe onload issues, I found note
in http://developer.yahoo.com/yui/releasenotes/README.connection and
that 2.5.2 issue description gave me idea how to fix this bug:
attach onload handler after iframe is inserted into DOM!

After applying following patch, form plugin works flawlessly under Opera (9.27, 9.52, 9.64, 10.00), I also tested (without problems) IE6,7,8 and Fx2,Fx3.

diff --git a/public/javascripts/jquery.form.js b/public/javascripts/jquery.form.js
--- a/public/javascripts/jquery.form.js
+++ b/public/javascripts/jquery.form.js
@@ -261,7 +261,9 @@ $.fn.ajaxSubmit = function(options) {

                                // add iframe to doc and submit the form
                                $io.appendTo('body');
+                               setTimeout(function() {
                                io.attachEvent ? io.attachEvent('onload', cb) : io.addEventListener('load', cb, false);
+                               }, 1);
                                form.submit();
                        }
                        finally {