So, as Hunter linked to me and according to IGN, the Horse Update is due out July 1st (full release notes here until the patch is made final, at which point this link won't be applicable). Some mechanics will be changing, namely the controls used to direct vehicle objects like boats and horses, among other updates. Also, quite a few new blocks will be added, and the use of donkeys and mules might give a more Dungeon Siege feel to Minecraft.
As usual, we will not be updating the server until Bukkit supports the 1.6 "horse" update, which may take a few weeks following release. If you do update, try to save a copy of your old files somewhere useful, because the new launcher will supposedly provide an option for reverting Minecraft to an earlier version.