Well first (much as I may hate to admit it) Bumi is a better earth bender then Toph because he could bend while in that metal cage. If you remember back to the blind bandit when Aang and Toph were captured she couldn't escape from the cage and I doubt she would just let them take her hostage.

My only comfort at this point is what Pooldude said in a different thread.?? The creators went to a lot of trouble to introduce Toph and develop her have us get to know her.?? It would be a waste of their time to then just suddenly drop her and write her out of the story.?? I'm sure we'll see more of her.?? In what capacity, I don't know but I'm sure we'll have her around again.?? She's too good of a character anyway just to toss aside.?? That's my 2 cents!