<p>What's the best way to run scheduled tasks in a Rails environment? Script/runner? Rake? I would like to run the task every few minutes.</p>

Assuming your tasks don't take too long to complete, just create a new controller with an action for each task. Implement the logic of the task as controller code, Then set up a cronjob at the OS level that uses wget to invoke the URL of this controller and action at the appropriate time intervals. The advantages of this method are you:
Have full access to all your Rails objects just as in a normal controller.
Can develop and test just as you do normal actions.
Can also invoke your tasks adhoc from a simple web page.
Don't consume any more memory by firing up additional ruby/rails processes.