The fact that Apple choose to stick with the original iPad storage options for iPad 2 (16GB, 32GB and 64GB) clearly shows that the iPhone 5 will unlikely get more than the existing 32GB on top of the 16GB option already on the market.
The spot price of Flash NAND memory is still quite high with 16GB costing $32 and 32GB likely to cost around $64. Even if Apple gets a significant discount, 64GB worth of memory could account for up to a third of the bill of material of the iPhone 5.
That's more than the cost of the processor, the display, the camera, the battery and the baseband chip put together - assuming that costs haven't risen since the iPhone 4.
Also the increasing reliance of smartphones on cloud storage means that there is less impetus to add more onboard storage; there are rumours that Apple may be looking at offering MobileMe, its online services for free while significantly expanding the scope of the service to include in-cloud syncing.
Ditto over the ongoing rumour of iTunes being moved to the cloud, one that has been around since Apple acquired Lala.com in December 2009.