I think profit margin is the determining factor in whether or not the next gen iPod will have a touch screen or not. How much does the touch screen cost? Is that the biggest cost of the iPhone or is it other technologies? (I really don't know) I think they've got near 50% profit margin on most of their products, unless I'm mistaken.
If they can make a new iPod at the same price points and increase their profit margins at the expense of a touch screen, then I believe they will do so. If the touch screen does not drive the cost up (i.e., smaller profits) then they will incorporate it. It seems to me a financial decision at this point and has nothing to do with cannibalizing iPhone sales or moving the iPod technology forward. Until their profits drop on the current iPod design, there is no reason changing anything radical unless their market-share is in jeopardy. Look at the mobile computer market. Has the MBP design changed significantly since the aluminum powerbooks? Eliminating 12" laptops and having the consumer line all one size was more of a financial decision than an esthetic one I believe.
Never forget Apple wants to make money - and they've done that very well. Something stock holders like too