onload not called on window load

[Adam Barth]

If a page define an onload function:

window.onload = function () {
  alert('onload called');
}

Is onload called when the page loads?

Firefox 2: Yes
Internet Explorer 7: Yes
Opera 9.24: Yes
Safari 3: Yes

If a page defines the same function using a different syntax:

function onload() {
  alert('onload called');
}

Is onload called when the page loads?

Firefox 2: Yes
Internet Explorer 7: No
Opera 9.24: No
Safari 3: No

Seems like onload should be called in both cases.