Currently Tomcat 6.0.26 is bundled with the binary distribution of Pluto (both 2.0.x and trunk), which is too old.
I've tested against latest Tomcat 7.0.21 and everything works fine, including validating against the Portlet 2.0 TCK.
One specific and major change is needed however for upgrading to Tomcat 7.x:
The emptySessionPath="true" server connector setting no longer is supported! (see also
The new solution is to add sessionCookiePath="/" to the root context.xml ($CATALINA_HOME/conf/context.xml) Context element,
e.g. change <Context> to <Context sessionCookiePath="/">
See also: http://tomcat.apache.org/migration.html#Session_cookie_configuration
And also nice: it is already support from Tomcat 6.0.27+ which means this new configuration can be used for any latest Tomcat 6.x as well!
Other than this change and the version bump to Tomcat 7.0.21 (both done in dist-build.xml) nothing else is needed to upgrade to latest Tomcat 7.x