I had one of these for my 2G as well, but switched to a Beyza sleeve case for my 3GS. While I liked both cases, I've decided to go ahead and preorder the UltraSlim for my iPhone 4 as I really like how minimalistic it was. While I wish it would arrive before my new phone arrives, I'll temporarily keep it in another case until Sena is able to ship my order.
My guess is that the case will be generally the same, but tailored for fit the new model snugly.