I have not seen ipp files elsewhere, but Boost libraries puts implementation of templated code in ipp files instead of in cpp files ( http://lists.boost.org/Archives/boost/2003/08/51197.php ). I have defined it in my .ackrc file but since Boost contains many widely used C++ libraries, I am proposing supporting ipp files under cpp group by default.
Originally posted in https://groups.google.com/forum/#!topic/ack-users/ya-8rBCun0E
Reposting here as requested by @petdance