Feel free to correct me if I am wrong, which I am sure someone would be ecstatic to do anyways, but Apple treats the refurbs pretty much like new computers. You get the full warranty and the latest revision of normally bundled software, ie iLife and Mac OS. So once 10.5 is released I believe it will be be installed on those units which are refurbished after release, and those refurbished before the release but sold to the customer after release or after grace period Apple specifies will either get the OS disc included or you get the upgrade shipped to you. Usually there is a grace period of purchases made within a few weeks of a new OS for free upgrades, probably so Apple is not inundated with refunds for returned computers.
Just like if you had gone to say CompUSA after 10.4 was released but had gotten a Mac that had been sitting on the shelves for a while and still had 10.3 installed. You could get the upgrade to 10.4 for free. Perhaps paying shipping.