It built for me (5.1.0.GA) and the only difference I can see is I am using Maven 2.2.0.

Which sources did you use? The tar.gz file form the downloads page or did you checkout from Subversion?

I used the tar.gz file from the web page.

1) What OS are you running?
2) What is the full path name to the base source directory?
3) Do you have internet access with no firewall restrictions?
4) Did you verify the download using md5sum?
5) Just to verify: opened a command prompt, cd'ed to the src/build directory, and then ran build.bat
6) How far down the build did it get? In other words, do you get this error almost immediately or does the build go for a while before getting this error? Do all of the third party JARs get downloaded (Explorer tells me that there are 2382 files and 535 folders in the thirdparty directory)

I haven't recently (or rather never) built the source from the downloaded versions. But, shouldn't users change their maven settings.xml to include the JBoss maven repos as explained here http://www.jboss.org/community/wiki/MavenSettings

I double-checked my m2 settings and I don't have the JBoss Maven repository there. I would suspect that the repository is specified in the POMs that need it.