a tag as a submit button?

Give the form an id, and then:

document.getElementById("yourFormId").submit();

Best practice would probably be to give your link an id too, and get rid of the event handler:

document.getElementById("yourLinkId").onclick = function() {
    document.getElementById("yourFormId").submit();
}

Try this code:

<form id="myform">
  <!-- form elements -->
  <a href="#" onclick="document.getElementById('myform').submit()">Submit</a>
</form>

But users with disabled JavaScript won't be able to submit the form, so you could add the following code:

<noscript>
  <input type="submit" value="Submit form!" />
</noscript>

Supposing the form is the direct parent you can do:

<a href='#' onclick='this.parentNode.submit(); return false;'>submit</a>

If not you can access through the forms name attribute like this:

<a href='#' onclick='document.forms["myform"].submit(); return false;'>submit</a>

See both examples here: http://jsfiddle.net/WEZDC/1/