Prevent Form Submission With Dynamic Onsubmit?

I'm trying to create a form whose onsubmit returns false if the form
should not be submitted. Normally this type of thing works great for
me:

<form onsubmit="return somefunction()">

where somefunction returns false. But I'm trying to do some thing where
in some
script block, the function gets added:

form.attachEvent("onsubmit", function () { return somefunction()
});

(not worrying about crossplatformosity, sorry). But that doesn't work,
the form is always submitted. Is there something else I need to be
doing to make this happen?