How do I create a Forum topic post?
Er, yes, I just succeeded in creating a Forum topic, after many trials. This was successful :

curl -i -X POST \
  -H Cookie:SESSe6a353351df0bfa8deadcafe=B0ZkiW3-U2x93yJU-deadbeef \
  -H X-CSRF-Token:PSCd5qIdddax0Mh5gfnOM872hdj893-eeeeffff \
  -H Accept:application/json \
  -H content-type:application/json \
  http://myserver/rest/node \
  -d '{
    "type": "forum", 
    "title": "test post?", 
    "language": "und",
    "taxonomy_forums": {
      "und": "1"
    },
    "body": {
      "und": [{
        "value" : "test post",
        "summary": "this is a test1",
        "format": "full_html"
      }]
    }
  }'


The key difference was, I think, adding this to the JSON: 

    "taxonomy_forums": {
      "und": "1"
    },


I guessed this, based on a view of the Firefox firebug Net panel, when using the UI to post a new forum topic. This number specifies the forum to which the new topic should be associated. It is a different matter to get the list of forum topics!

Maybe this will help others.