step-keep.js

var getFiles = Step.fn(
  function readDir(dir) {
    this.keep(dir);
    fs.readdir(dir, this.parallel());
  },
  function readFiles(err, dir, results) {
    if (err) throw err;
    // Create a new group
    var group = this.group();
    results.forEach(function (filename) {
      if (/\.js$/.test(filename)) {
        fs.readFile(dir + "/" + filename, 'utf8', group());
      }
    });
  }
);

getFiles(__dirname, function(err , files) {
  if (err) throw err;
  console.dir(files);
});

Merging this and this.parallel() would be fantastic. The fact that you'd have to use this.parallel() when using this.keep() was a concern of mine, since it'd be a potential pitfall for users.