The grunt.registerHelper API has been completely removed from the 0.4.0a (devel branch) -- see issue 354 / changeset for the explanation.
An example of rewriting a task to remove registerHelper is here
Here's another look a the lint task - 0.3 to 0.4
Furthermore, grunt.utils has been renamed to grunt.util
compatibility with 0.4.0a
* grunt.utils -> grunt.util
* helpers removed entirely