Be aware that the tar program shipped with AIX is not GNU-tar compatible, so you may want to download the .zip distribution and use $JAVA_HOME/bin/jar -xvf to decompress it.
Make sure you have the latest IBM JDK filesets installed since we have hit many bugs in previous releases. I think the latest as of today is:
IY75003 ca142-20050929a 126.96.36.199 142 SR3
Also, I'd suggest building JBoss with the IBM JDK.
Other than that, it works fine for us.