I doubt it, but you can use javascript to have an a tag submit a form:
Code:
<a href="#" onclick="document.forms.formname.submit(); return false;" class="css3dbutton">Sign Up!</a>
Where formname is the name of the form you want to submit.
Of course this will not work if javascript is not available on the client's browsers. Most have it, but there should be a non-javascript fallback:
HTML Code:
<form name="formname" action="#">
<!-- The rest of the form would go here -->
<script type="text/javascript">
document.write('<a href="#" onclick="document.forms.formname.submit(); return false;" class="css3dbutton">Sign Up!</a>');
</script>
<noscript><input type="submit" value="Sign Up!"></noscript>
</form>
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