drupal 7 + services 3 - inserting nodes with select fields
Eventually I figured this out:

    {
    "title": "some more tests",
    "type": "weed",
    "field_sprouts": {
        "und": [ 8,9 ]
    },
    "field_urgency": {
        "en": [
            {
                "value": "2"
            }
        ]
    },
    "field_species": {
        "und": [ 4 ]
    }
 }


species and sprouts are entity references and only need the nid of the target (no value= or target_id= or anything).
For unknown reason (as of yet), the urgency field required a language to be defined (other then und[efined]). Its the only select field here with a normal select list, not entity refernce.
Make sure your widgets are select list and not auto complete.