I think external expansion is a no no with Apple.
firstly it doesn't allow them to tightly control the cards people will use, and that slow cards might give off a perception that it's the iphone/ipod that's slow, and therefore the product is less than optimal.
Also, If you can expand it, then how will they sell you a new one when you fill it? Seems that the conspicuous lack of an expansion port is a very deliberate choice, and if apple wanted to add one, they'd have done it a long long time ago.
I fear battery life will suffer if 3g is used. I know a couple of people with 3g phones and when they get online with the 3g goodness, the battery life is pretty poor.
But you never know with apple. Won't be cheap though, that's for sure ;/