Not that it's exactly the answer to the question, but window.onload is unreliable. It works MOST of the time with IE, but FireFox and Safari offer really flaky support, while Opera and others don't support it at all.
The best solution I've come accross for telling if the DOM Content is all loaded, is this:
I use it on a lot of my pages that contain Telerik controls, so I know when the page is loaded and safe for me to start modifying the DOM... and I'm yet to have a problem with any browsers I've encountered.
As I said... not really the answer to the question, but something perhaps Terlerik could look at, and including something similar into the API as a standardised function.