The idea for the scroll wheel was suggested by Apple's head of marketing, Phil Schiller, who in an early meeting said quite definitively, "The wheel is the right user interface for this product."
Schiller also suggested that menus should scroll faster the longer the wheel is turned, a stroke of genius that distinguishes the iPod from the agony of competing players.
Jobs' role is best described like this (from History of the iPod: http://lowendmac.com/orchard/05/origin-of-the-ipod.html#0)
Steve Jobs took a very active role in the project, scheduling frequent meetings with the directors from Fadell's group and PortalPlayer. During these meetings he would tell them in detail what issues he had with the device, whether it was the interface, sound quality, or the size of the scroll wheel.
This was rare for an Apple project at the time, and it reassured the leaders in the group that the project would not be axed immediately.
So while Jobs was absolutely involved and an influence he didn't "design the interface"