I tried to confirm but everything went well. From my logs:
Setting up tomcat6 (6.0.28-2ubuntu1) ...
Creating config file /etc/default/tomcat6 with new version
Adding system user `tomcat6' (UID 105) ...
Adding new user `tomcat6' (UID 105) with group `tomcat6' ...
Not creating home directory `/usr/share/tomcat6'.
* Starting Tomcat servlet engine tomcat6 [ OK ]