Martin's comment was very helpful to me, but I found that further configuration is required if using jcl-over-slf4j because Jetty exposes the standard Commons Logging API to applications.
Revised configuration:
<webAppConfig>
<contextPath>/test</contextPath>
<serverClasses>
<!-- Copied from WebAppContext with SLF4J removed -->
<filter>-org.mortbay.jetty.plus.jaas.</filter>
<filter>org.mortbay.jetty.</filter>
</serverClasses>
<systemClasses>
<!-- Copied from WebAppContext with Commons Logging and Log4j removed -->
<filter>java.</filter>
<filter>javax.servlet</filter>
<filter>javax.xml.</filter>
<filter>org.mortbay.</filter>
<filter>org.xml.</filter>
<filter>org.w3c.</filter>
</systemClasses>
</webAppConfig>
Apologies for bumping an old, closed bug, but I found this bug when Googling for the problem, and expect this information may be useful to others.
Oops, I mean the error is "NoClassDefFoundError: org/slf4j/LoggerFactory" since it's slf4j-api that isn't visible.