Oddly in IE if you have a form with only one form element it won't click the button when you press return... I think you may be able to add a hidden form element to make the button click happen as well it is an odd issue with IE.
Would it be possible for you to give an example of the hidden form element that you'd put into the code to make this work?
I've been trying to find a way around the same issue as raised in this thread to no avail.