Here is the whole thing I'm sending to the API client: (Is it because I'm using the ResourceLink typename?)
$Fields = array(
'Resource.UserName',
'WorkItem.Name' ,
'WorkItem.CreatedOn',
'WorkItem.C_WorkStatus');
$Paging = NULL;
$createdOn = "2014-04-01";
$LeftDate = array (
"FieldName" => "CreatedOn");
$RightDate = array (
"Value" => new SoapVar ($createdOn, XSD_DATETIME, "dateTime", "http://www.w3.org/2001/XMLSchema"));
$DateCondition = array (
"LeftExpression" => new SoapVar ($LeftDate,SOAP_ENC_OBJECT, "FieldExpression", "http://clarizen.com/api/queries"),
"Operator" => "GreaterThanOrEqual",
"RightExpression" => new SoapVar ($RightDate,SOAP_ENC_OBJECT, "ConstantExpression","http://clarizen.com/api/queries"));
$Where = new SoapVar ($DateCondition,SOAP_ENC_OBJECT, "Compare", "http://clarizen.com/api/queries");
$clarizenReturn = $clarizenAPI->getList('RegularResourceLink', $Fields , $Paging, $Where);