I recommend you all use also the greate java forums-
it's friendly and you get the answers real quick.
The answer for that one is:
Click on "view all files", and then go to "JBoss" - "JBoss-5.1.0.GA".
I'm not sure why the downloads are hard to find:
In the tabs at the top of the page hover over Projects and then move the mouse to Servers and the Application Server and click.
Then click on the Downloads button in the blue box on the left-hand side of the page.
Then click the Download link for the 5.1.0.GA entry. Then click the boss-5.1.0.GA-src.tar.gz link.
(A little while ago the SourceForge site went through some redesign and the download links were very difficult to find, but it looks like that has been fixed.)