Elliotte Rusty Harold had an excellent suggestion for PMD - provide a ruleset that lists the rules that are new in each release. Done and done, now you can run PMD and get a nice list. For example, to see what rules were new in PMD 3.4, just reference the rulesets/releases/34.xml ruleset:
and for the latest release, 3.5, simply change the ruleset name:
The same thing works with the Ant task; just run Ant with the -v flag. I've added "release rulesets" back to PMD v3.3, and I'll continue to fill in the gaps for previous releases. Good times!