Apparently, IE7 fails to submit the form with the default button if the form only contains one text input box. Here is the fix for that. Just create a text input in a hidden div on the page. This will circumvent the IE bug.
Example div:
<!-- Fix for IE bug (One text input and submit, disables submit on pressing "Enter") --> <div style="display:none"> <input type="text" name="hiddenText"/> </div>
Also see: http://stackoverflow.com/questions/270494/enter-button-does-not-submit-form-ie-only-asp-net