Starting with the gem for 0.13.pre, the permissions are wrong in the tarball inside the gem itself. I did this by installing all the released versions on rubygems into a local directory and running ls -l on them. Annoyingly manual, but it's done now..
Whoever is packaging the gems has bad ownership flags set on their checkout and it is propagating to the released gems.
Sam, did something happen to your computer around the end of August (based on release dates)?
chmod -R +r Ruby/ in the checkout before releasing the next version should fix it, I think.
This would never show up for RVM users, since the gem would be owned by the user. Also, production environments using bundler, because the gems are installed locally.
sudo chmod -R +r /var/lib/gems/1.9.1/gems/rack-mini-profiler-0.1.21/ after installing made the error go away.