Are you running standalone or domain? Have you looked at the scripts used to start AS 7? If you did, you would see that it uses -jar rather than identifying a main class. (If you look in the jar file its manifest identifes a main class.) Therefore I suspect you would change you <java> task to use the "jar" (and not the "classname") attribute. In addtion, you will also need to pass the other parameters that are new to AS 7.
If you need more help than that, post the full task (or better yet, the whole target) used to start JBoss AS.