gruntjs/grunt-contrib-concat — pull request #45 by uberbrady (2013-09-03 23:47:32+00:00)
Title: Clarify name of the data object for 'process'

Grunt.template documenation seems to imply that you should set your options like:

options: {
data: {
myvar: "blah"
...

etc.

And that seems to work for me.