I have a similar problem. I'm trying to update a field (Research_Reason__c) on a custom object (lda_Opportunity__c). I can query a record fine, but updating is a little different and I can't figure it out.

Here is my function:

function FlagFixDeal($id,$reason,$researchtime){
$access_token = $_SESSION['access_token'];
        $instance_url = $_SESSION['instance_url'];

$content = json_encode(array("Research_Reason__c" => $reason));
$url = "$instance_url/services/data/v20.0/sobject/lda_Opportunity__c/$id";

    $curl = curl_init($url);
curl_setopt($curl, CURLOPT_HEADER, false);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_HTTPHEADER,array("Authorization: OAuth $access_token","Content-type: application/json"));
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "PATCH");
curl_setopt($curl, CURLOPT_POSTFIELDS, $content);

curl_exec($curl);
$status = curl_getinfo($curl, CURLINFO_HTTP_CODE);

if ($status != 204) {
  die("Error: call to URL $url failed with status $status, curl_error " . curl_error($curl) .", curl_errno " . curl_errno($curl));
}

echo "HTTP status $status updating deal.<br /><br />";
curl_close($curl);
}

Error I'm getting is:
Error: call to URL https://na19.salesforce.com/services/data/v20.0/sobject/lda_Opportunity__c/a0Oa000000LAkarEAD failed with status 404, curl_error , curl_errno 0

Thanks in advance for any help you can provide.