Modify dinamically <title> data

I am to modify the <title> in about 100 pages. The Head section of each page is in fact a sigle php include. Unfortunately I have to have different titles for groups of pages.

I tryed to change dinamilically the <title> using:

document.documentElement.getElementsByTagName('title')[0].data;
or
document.documentElement.getElementsByTagName('title')[0].appendChild(document.createTextNode('test'));
or
document.documentElement.getElementsByTagName('title')[0].innerHTML

with no effect

The assignement is located inside the body. Nomatter it is a simple assignment or called onload in afunction, it looks like the head's tag can not be changed dinamically. Any suggestion?

BUG: Script Fails to Change the Document Title of a Web Page (http://support.microsoft.com/kb/q296113/)

Yes, thanx. I solved the problem by returning to the old DOM 1 Level document.title wich is working ok either in IE6 and Moz, which is enough for my client. I have noticed that DOM2 Level methods are not working upon TITLE tag... hm.. could be a bug or rather a sort of security enclusure, I don't know