The classpath used to compile JSP files seems to include all files and directories under the deploy dir (and even more). I turned on Trace level debugging for category 'org.jboss.web.tomcat' and looked at the trace level output. Each time a file is deployed, it is added to some list that is used as this classpath. This is even the case for .xml files!
This seems inefficient. Even worse, when a .war file is deployed as a one of the last items, the JSP compile classpath can potentially have become so big that the java compiler fails to compile with a classpath too large error message.
Note: I'm using jboss-3.2.8SP1, haven't tried yet under jboss-4.0.x.