To run grunt in debug, you need to pass the grunt script to node explicitly:
node-debug $(which grunt) task
and put a
debugger; line in your task.
node-inspector will then open a browser with debugging tools.
Edit 28 Feb 2014
node-inspector has added the command
node-debug, which launches node in a
--debug state and opens the browser to the
node-inspector page, stopping when it hits the first
debugger line or set breakpoint.
Edit 30 January 2015
On Windows, things are a touch more complicated. See the answer from @e.gluhotorenko for instructions.