Looks like the download link is broke today. Sure hope someone from JBoss is monitoring this forum...
The SourceForge download links are having some issues because of their recent software upgrade. All JBoss downloads (including source of JBossAS) are available on a single large page (which makes it difficult to locate the exact download) at http://sourceforge.net/projects/jboss/files/
For 5.1.0.GA source code, the URL is http://sourceforge.net/projects/jboss/files/JBoss/JBoss-5.1.0.GA/jboss-5.1.0.GA-src.tar.gz/download
And as Peter mentioned, you can even use the SVN to checkout the source from the tag.
The SourceForge links are still broken. I didn't need the source code, but browsing directly to the directory you mention does the trick to access all possible files. Thanks.