Could it be because of id-mangling? ASP.NET feels that it can freely change your IDs for tags with runat="server". Check your generated HTML and check the ID of the form.
To solve that particular case, you can add a wrapping element that does not runat=server to the form, and target the form throught that one.
<div id="myform_wrapper">
<!-- your form here -->
</div>
and target it with
$("#myform_wrapper form")
Magnar
2009-03-15 17:28:30