Trying to create a user with an entity reference taxonomy term using services
I was able to get it working with the following on checkboxes and select:

"field_likes": {
"und":{
"target_id": "1"
      }
}


This technically solves the problem, though I'm not sure how to send up multiple values this way.

EDIT:

Here is the actual solution, which not only allows you to put in entity reference taxonomy terms, but multiple as well:

"field_likes": {
                 "und":{
                         "target_id": "1",
                         "target_id_1": "2"
                        }
                }


This is with the select list widget, which I think is important for this particular example (based on other links I've been reading).