[griefon]

Вот вам пример:

ConditionExpression cond = new ConditionExpression();
cond.AttributeName = "scheduledstart";
cond.Operator = ConditionOperator.Today;

FilterExpression filter = new FilterExpression();
filter.Conditions.Add(cond);

QueryExpression query = new QueryExpression();
query.Criteria = filter;
query.ColumnSet = new ColumnSet("phr_invid");
query.EntityName = "serviceappointment";

EntityCollection appointments = service.RetrieveMultiple(query);

Код под 2011, но под 4.0 почти тоже самое.