Maybe this is just a documentation problem. Googling around suggests that Jetty wants you to place any "extra" JARs not in $JETTY_HOME/lib but in $JETTY_HOME/lib/ext. I just dropped the 1.0.6 JARs there and jetty started without issue and now I have request logging going.

bobk-mbp:jetty bobk$ ls -l lib/ext/
total 864
-rw-r--r-- 1 bobk staff 89638 Jun 22 08:48 logback-access-1.0.6.jar
-rw-r--r--@ 1 bobk staff 349706 Jun 22 08:48 logback-core-1.0.6.jar