I've actually had a conversation about the problems with using PlanetLab infrastructure for testing discrimination recently.
The Planetlab network is comprised of servers deep within University networks.
A nice list of sites participating in Planetlab can be readily found here: »www.planet-lab.org/db/pub/sites.php
My dedicated boxes get polled by planet labs servers all the time; I grabbed a few from the logs that were obvious choices:
Traceroute to those servers and you will notice they are far beyond the border routers of the Unversity.
Why is that significant?
Colleges throttle traffic they believe to be non-critical to academic purposes -- they're open about doing so. Nearly every campus has rate limiting in place to ensure academic network uses are not impacted by recreational traffic.
PlanetLab will undoubtedly find many cases of traffic prioritization, but it will most often occur on the network their test boxes are directly attached to. This testing is no more valid than the tests performed by the researchers at the University of Colorado
where they found "evidence" of ISP interference by DoSing their own campus network's NAT gateway.