As updated on the Pokémon Ranger sub-site, the final mission is being released to the Ranger Net.
The mission name: 'For Bride and Shaymin'
The final mission requires you to have beat the game first. Also, you won't be able to send Shaymin to your Diamond, Pearl, or Platinum game.
It will be available from March 1st to April 30th.
Of course, the Ranger sub-site can be found by clicking here.