The decive is really too small for HD. The smaller the device, the more likely the user is to shake. On a large screen HD screen, it would look horrible.
So would you go the expense of a HD camera to view on the iPod Touch's own screen, probably not.
There is also the small issue of the processor, heat and battery life. Capturing and encoding HD video is extremely processor intensive. Even if the processor coped, it would get very hot, which is difficult to dissipate in such a small device and would require a larger capacity battery than the current one.
Sometime in the future, it may happen, if the iPod Touch still exists.
Phil