William, currently 24, has previously stated that he does not intend to marry till he hits 28 but the fact that the titles have been 'reserved' is somewhat of an indication of the thoughts of the Queen on this matter.
The title would only be a temporary one however. Once his father ascends to the throne, William would inherit the title 'Prince of Wales' and his wife would become the 'Princess of Wales'.
If you do happen to be reading this young master William, then please remember to send us an invite (fat chance I know, but if you don't ask, you don't get!).