OK, i think i've solved my questions. I've been able to run a Java Web Start application from a Tomcat server (version 6.0.13). So...
1. Yes, JBoss can handle JNLP files, since it includes a Tomcat server.
2. The .jnlp extension is set by default to the application/x-java-jnlp-file MIME type in Tomcat (the binding is specified in conf/web.xml).
I managed to deploy a simple Java Web Start application in JBoss. Here's how i did it. (Note: i use JBoss 4.0.5 and JDK1.6.0_02).
1. I created the folder "myWebStartTest.war" under the "/server/default/deploy/" folder.
2. I copied to myWebStartTest.war two files:
* the example JNLP file listed in the Java Web Start tutorial at http://java.sun.com/docs/books/tutorial/deployment/webstart/deploying.html
* the "Notepad.jar" file from the \demo\plugin\jfc\Notepad directory within the JDK installation directory.
3. Inside the Notepad.jnlp file i changed the value of the "codebase" attribute of the root element to "http://XXX:8080/myWebStartTest/", where XXX is my server's IP address.
4. I created a "index.html" file containing a link to "Notepad.jndl" (i didn't specify any path, simply "Notepad.jndl").
And that was it. All that was left to do was to run JBoss (actually, JBoss could have been running all along) and point my browser at http://XXX:8080/myWebStartTest/ .
Hip hip hurray to me!