My guess is there will be little to no effort made to mod in parts. Many games that have contained the possibility of adding parts or similar stuff, galciv2 for instance, have little if any mods made that add stuff like that. People seem to prefer adding original whole creations, like a new model to select from in the ship creation tool that contains a pre-made enterprise (yes I'm aware you can build it anyway) or other such things. Now because in spore you can build so much it leaves very little room to mod. Parts, new planet dynamics, etc.
Anyway I'm guessing that very little finished mods will actually come out for Spore because A) The mods possible don't really appeal to people B) The difficulty involved in modding.
Then again, I could be wrong and they could design the game to be easy to mod but I'm betting they won't put much effort in that, because then what will they include in expansion packs if they don't add more parts? Lets be serious here people, the expansion packs might end up being likes The Sims and contain only 1 game mechanics change per game.