Enter key wont submit form on Chrome/IE but does on Firefox

[riskmod]

Anyone know what up with this??:

<form id="form1" action="find-a-doctor" method="post">
....
...

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['form1'];
if (!theForm) {
theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
theForm.__EVENTTARGET.value = eventTarget;
theForm.__EVENTARGUMENT.value = eventArgument;
theForm.submit();
}
}
//]]>
</script>

<input type="submit"  id="main" class="btn-submit" " value="" name="Search">

....
..</form>

The class had the graphic bg...again its fine in FF but in Chrome/IE7+ ....I also tried <button type...............