Hi

I'm running the sample app from here http://wiki.developerforce.com/index.php/Getting_Started_with_the_Force.com_REST_API

The following code successfully creates a new Account:

PostMethod post = new PostMethod(instanceUrl
				+ "/services/data/v20.0/sobjects/Account/");

However, when I try to create the following custom object, I get a 400 error:

PostMethod post = new PostMethod(instanceUrl
				+ "/services/data/v20.0/sobjects/OpportunityAccount__c/");

I can see the custom object listed when I issue: 

GetMethod get = new GetMethod(instanceUrl
				+ "/services/data/v20.0/sobjects");

I get back the following: 

 {
      "activateable": false,
      "createable": true,
      "custom": true,
      "customSetting": false,
      "deletable": true,
      "deprecatedAndHidden": false,
      "feedEnabled": false,
      "keyPrefix": "a00",
      "label": "OpportunityAccount",
      "labelPlural": "OpportunityAccounts",
      "layoutable": true,
      "mergeable": false,
      "name": "OpportunityAccount__c",
      "queryable": true,
      "replicateable": true,
      "retrieveable": true,
      "searchable": true,
      "triggerable": true,
      "undeletable": true,
      "updateable": true,
      "urls": {
        "describe": "/services/data/v20.0/sobjects/OpportunityAccount__c/describe",
        "rowTemplate": "/services/data/v20.0/sobjects/OpportunityAccount__c/{ID}",
        "sobject": "/services/data/v20.0/sobjects/OpportunityAccount__c"
      }

Would appreciate any advice on what I may be doing wrong. 

Thanks