As Unspoken Demise said,
Most likely they have the feature available, but want to save something for next year (Like they did with the iPhone/3G).
However, it is also possible that they couldn't fit 4 x 8GB flash chips on the circuit board, as well as a camera, and keep it the same size. They'd probably have to make it fatter, which you know Apple WON'T do.
Like even with the 64GB Touch. They're probably using either 8GB or 16GB flash chips in there, they want to keep costs down as every business does, so they don't use 32GB chips (v.expensive and rare?)