First, you can run JBoss 4.0.5GA using JDK 1.5.0_11. And actually it is recommended since 1.5.0_11 has the latest fixes. (This is especially important if you are running on Vista.) The recommendation of 1.5.0_06 was the minimum patch level for 1.5.0 that is acceptable.
Second, you can develop applications using JDK 1.5.0_11 and run them using JDK 1.5.0_06, because they are both 1.5.0. But you cannot develop applications using JDK 6 and run them with any 1.5.0, if you do you will get a class version number exception.
For Ant, you can use whatever version you want to build your apps. If you build JBoss itself, it comes with its own version of Ant, so that should not be an issue.