jamesqua:The answer to your second question is yes.
It will replace any other functions hooked to that event. Here is a
way to
attach multiple events. Perhaps there is a better way to go about this. What exactly are you trying to achieve?
Thanks, this is exactly the answer to what I was asking about.
NC01: 
What are you going to sue me if it doesn't? There is no way to guarantee of that. That is just not the way browsers work.
No, I'm not american - I'm not into sueing. 
The reason I was asking if "you guarantee" is because I heard from several people that attaching a script to the onload event doesn't guarantee that the page (with all its elements) is going to be fully loaded by the time the script executes. So rephrasing the "you guarantee" would be "does it usually/always work, from your experience?"
NC01: So just put all other scripts/functions that you want in the onload in there.
Unfortunately, that is not always possible 
For example, in my case, I'm dealing with rather large e-commerce application that uses xml templates to generate web pages.