I use this to invoke a function when a form on the page is submitted:
$$("form").invoke("observe", "submit", submitForm);
I'm having a problem getting this to work in IE when a text field has focus and the enter key is pressed. Firefox submits the form in this case but not IE.
The form has one submit button:
<input type="submit" value="Submit"/>
Clicking the submit button works fine in both browsers using this method.