Recently-revealed engineering samples of iPhone 4s have been found to have 64 GB of memory -- see picture above from Unwire, and this M.I.C. Gadget story -- showing that there's no physical reason why the iPhone 5 couldn't have this much memory.
The iPad 2 has already been criticised for sticking with the earlier version's 64 GB limit, and releasing an iPhone 5 with the earlier model's 32 GB limit would be bound to increase the complaints. Previous speculation about the 32 GB limitation on the iPhone's memory capacity centered on lack of physical space, production capacity and cost. With capacity due to almost double this year and prices falling, it looks like the third barrier may now have fallen if Apple can secure enough supplies.
The 64 GB engineering samples are very similar to the (in)famous Gizmodo iPhone 4 from last year (with a similar serial number) and appear to date to early 2010, showing that it was indeed constraints other than physical ones which kept Apple from releasing an increased-capacity iPhone 4. It was apparently obtained from sources at Apple manufacturer Foxconn rather than on the Hong Kong grey markets - you can watch a video of it in action here.
Update: Engadget spotted a 64 GB iPhone prototype in Hong Kong's Sin Tak shopping mall. Upon investigation it appears as though it was made in 2010 but the serial numbers were invalid. Perhaps if prices on memory drop the iPhone 5 could still sport a higher capacity.