The modular nature of Spore gives it a lot of potential for handhelds IMO...
For example, they could turn the 'petri-dish' episode into a full game on DS - and whatever creature you evolve there could ultimately be transferred onto another system (like PC) to continue its evolution in a different copy of the game
Edit: Knowing EA, though, they'll probably release one good version of the game (ie the real version), with a bunch of compromised ports. Then they'll have a random internal team knock out some dross for the handhelds, under the same brand name