I'm not sure is it appropriate, but I've tried to use Bulk.execute() without a callback and it gives me an error.

I'm using Meteor v1.2.1 and have the following code:

var bulk = Collection.rawCollection().initializeUnorderedBulkOp();
...
bulk.execute();

And it throws the following error:

(STDERR) TypeError: undefined is not a function
(STDERR)     at .../.meteor/packages/npm-mongo/.1.4.39_1.17b8oex++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/collection/batch/unordered.js:359:5

It appears to be a bug in this.execute = function(_writeConcern, callback): there is no check on callback and it actually requires this parameter to be defined and call it as a function.

Hope it helps!