It is difficult to include more flash space on the iPhone...
The iPhone only gets one flash memory, while the iPod Touch gets two...
If they are going to make 64 GB chips, then expect the iPhone getting 64 GB of storage, with the iPod Touch getting 128 GB...
At this point, months after Japan's earthquake, I do not know if flash memory prices have stabilized, but they were getting quite expensive again...
In the end, wait eventually or get the iPod Classic (one of the many reasons it is still being sold)...