It is dependent on the availability of bigger solid state memory chips, NAND chips. From what I've read 32GB ones are coming, so these might allow for a 64GB touch
eg http://macblips.dailyradar.com/story/nand_flash_supply_to_tighten_after_apple_reportedly/
Current devices combine multiple 8GB ones, the link above claims Apple have ordered 100 million of the 8GB ones, quite a few, but then they sell a lot of iPods etc
this is a good place to keep an eye on new Apple releases
http://buyersguide.macrumors.com/