The iPod Touch is 64GB because it has 2 memory slots, and the iPhone only has one. It seems that when there is a new size of microSD card coming out, then the iPhone gets updated memory. I haven't heard of a 64GB microSD yet. I'm not ready for the iPhone 4, so it's easy for me to wait for next years model with 64GB.
microSD is not related to iPhone NAND developments.
Also, AFAIK, no other phone maker is using anything higher than 16 GB NAND in their phones (Palm being the only 16 GB I can think of).