Finally figured this out. The issue is with the 'turn' gem included in Rails 3.1, or actually with turn v0.8.2, which is required by the default Gemfile:
group :test do # Pretty printed test output gem 'turn', '0.8.2', require: false end
Turn v0.8.2 doesn't include the full backtrace, so you have to upgrade to get it. I did that by changing the above in my Gemfile to this:
group :test do # Pretty printed test output gem 'turn', require: false gem 'minitest' end
(I had to add minitest because otherwise turn throws a RuntimeError saying "MiniTest v1.6.0 is out of date.")
Then I ran
bundle update turn and got the latest verion (0.9.2 as of this writing). That gives full backtraces.