You see RTM at many places on Micrsoft web-site. Actually RTM is a Microsoft acronym for the date that a product is scheduled to be released to manufacturing, where manufacturers are OEMs like Dell, HP or MS Partners (who will propagate product further into chain).

So, if MS says X product is planned for RTM in June 07, you can plan to develop your product using X after that time. X would be a fully tested and released product by that time.