Modify the From section of email sent by WPMU to the user when a blog is created

Hi:

Still new to WPMU 2.8.4a.

When a blog is created an email is sent to the user with the following info:

Subject: [Sitename.com] Your username and password
From: "WordPress" <administrator@sitename.com>
To: urb3@email.com
Username: urb3
Password: b4f2e4f8
http://sitename.com/wp-login.php

I would like to modify the "From: WordPress" part. I want to modify WordPress for the site name.

Browsed a lot for the topic on WPMU Forums and can't find which file and which variable do I have to modify.

Any clues will be appreciated.

Thanks.