The Macbook might have the i5, who knows though. It is Apple after all. It just depends on what motherboard they use, since they will have to completely replace the Core2 based ones.
The i3s and i5s use a different micro-architecture than the Core2s. They are more efficient and faster on a per clock basis. The i3 at 2.26ghz will perform a little bit faster the the P8400 at the same speed. It's not a huge difference, so depending on what you intend on using the Macbook Pro for, may or may not be worth the wait.