Solved.
If you need to use slf4j in your server classes, you must "promote"
slf4j classes to system classes. You do that by configuring the
WebAppContext just like this:
<Configure class="org.mortbay.jetty.webapp.WebAppContext">
<Set name="systemClasses">
<Array type="java.lang.String">
<!-- we copied these paths from jetty WebAppContext source code ...
-->
<Item>java.</Item>
<Item>javax.servlet.</Item>
<Item>javax.xml.</Item>
<Item>org.mortbay.</Item>
<Item>org.xml.</Item>
<Item>org.w3c.</Item>
<Item>org.apache.commons.logging.</Item>
<Item>org.apache.log4j.</Item>
<!-- and ... added slf4j -->
<Item>org.slf4j.</Item>
<!-- we must promote slf4j to system classes, otherwise gwt
hosted mode will not allow loading them due to a policy
that don't allow server classes to be loaded from the
outside world (see gwt JettyLauncher source code). -->
</Array>
</Set>
</Configure>
You should put this file at /war/WEB-INF/jetty-web.xml. For more
information, see
http://docs.codehaus.org/display/JETTY/jetty-web.xml
Cheers,
Célio