Did you try setting the property "respectDocumentOrder" to true in your Jackrabbit Search Configuration ?
If true and the query does not contain an 'order by' clause, result nodes will be in document order. For better performance when queries return a lot of nodes set to 'false' (In 1.5 'false' is now the default).
You should set it to true really only if you need it, because query results will be iterated fully in Java in order to sort them.