Rails3: "rake test" task does not run tests.

Hello all,

I have a spiffy new rails3 app for which I can run tests individually,
but not via rake. Behold!

$ ruby -Itest test/unit/test_user.rb
Loaded suite test/unit/test_user
Started
.
Finished in 0.466033 seconds.

1 tests, 1 assertions, 0 failures, 0 errors, 0 skips

Test run options: --seed 21789

and

$ rake test:units --trace
(in /home/blt/Documents/projects/rails3apps/naughtyapp)
** Invoke test:units (first_time)
** Invoke test:prepare (first_time)
** Execute test:prepare
** Execute test:units

Then only possible peculiarity of this app is that I am using
DataMapper, rather than ActiveRecord. Here are the railties I am
including in application.rb:

require 'action_controller/railtie'
require 'dm-rails/railtie'
require 'rails/test_unit/railtie'

I must be doing something wrong, but I'm quite at a loss. Would
someone be so kind as to help me debug this issue?

Thanks,
Brian Troutwine