Yes i'm afraid we still have users using IE 6 and no matter how much I scream and shout about it, we need to support them!
I've experienced problems with IE in the past when more than 1 element has the same ID value, but the ID of the form i'm trying to submit is unique on the page.
This is the code i'm using to submit the form.
function submitForm(form_id, form_action)
var form = getElement(form_id);
if(form_action != "")
form.elements.action.value = form_action;
alert("There was an error.\n\nThe likely cause is that you are using Internet Explorer which generally doesn't follow the rules.\n\nYou should really be using Firefox.\n\nTo get Firefox please goto http://www.getfirefox.com.\n\nThe error generated was: "+error.description);