Although technically the ROKR is a Motorola product, this re-badged E398 candybar-style phone represents Apple's first foray into the cellular phone market.
Motorola and Apple worked together to bring Apple's iTunes music system to the ROKR, but the two companies were never able to work together in a seamless manner. Motorola didn't want to make a lot of changes in the E398 to accommodate music playback, and Apple didn't like the interface.
The phone used a 512 MB microSD card, but was restricted by its firmware to only allow 100 iTunes songs to be loaded at any one time. The reasons for the restriction are somewhat speculative, but it's likely that either Apple didn't want the ROKR to be competitive with its iPods, or the record labels didn't want music tracks making the leap from a controlled iPod environment to a cell phone device that was perceived to be more open.
The ROKR was a failure, but Apple learned some valuable lessons, lessons it would apply to an upcoming new product.