In case the recent rash of 128GB and 256GB USB drives are not enough for you Intel and Micro have developed a 34nm NAND Flash chip that allows them to squeeze another bit into each cell. This is a roughly 50% increase in density as current cells only allow for 2 bits.
the advent of the 34nm process will also allow for lower power needs and a smaller foot print for the actual drives.
However, there is a small problem, it seems that packing in the extra bit has lowered the cells reliability. This not something that you look at lightly. Still according to Intel and Micron the new chips are more than reliable enough for USB drives but not quite there for SSDs.
So be on the lookout for 1TB USB keys soon.