Apple are free to use nvidia's chips if they want to. Right now, they don't want to, because despite all the hype the tegra 2 turned out to be quite slow. The A5 GPU is much, much faster.
But anyway, you do realise that nvidia don't design the CPU part? They license it from ARM. Apple license the same design from ARM. Samsung and all the other tablet processor makers? They also license it from ARM. They all tweak it a bit, and add a GPU and other bits, that's all. As it is, apple is some way ahead on performance, why would they want to buy nvidia who are (as of right now) producing slower chips?
Also, this "It's going to have X cores" thing. For the CPU, yes, it's a good thing, but each core is likely to be similar in speed (or possibly slower!) compared to the current CPUs. More of them, but perhaps slower. Most apps don't use 2 cores yet, so what will 4 get you? It'll get you a bigger chip, that eats battery faster, and you probably won't see much speed improvement.
The GPU cores are somewhat meaningless, it can have 1,000 cores and still be slow, or it can have just 1 super fast one. All that matters is the speed it can make pretty pictures at, the feature set (so we devs can make the pictures prettier), how much power it needs, and most of all it needs plenty of software that will use it well.
There's very little software that uses the A5 GPU fully still. On android, there's likely even less that takes advantage even of the tegra 2. Again, more power might not get you much real benefit.
Last point on that: apple used imagination's SGX 543MP core in the A5. That chip is multi-core. Apple went with 2 cores, but they COULD have gone for 4 or even 8. But very little software will use even the power of 2 cores, and battery life would go down, so it seems they made the right choice, no?
Having said all that, I'm finding the ipad 2 somewhat underpowered for my current app, and wish it was quad core with a much faster gpu
