IMO they are all arrangers until you get to the Korg M3. To me the sine qua non of arrangers is CHORD RECOGNITION. If it cannot recognize and react to chord input in close to real time, it's NOT an arranger. The next main feature are styles: stored chordal patterns that are subject to transposition according to pre-determined musical rules. However, even a sustained triad (with or without the bass) can be thought of as a very simple style. This in turn could be used to trigger styles in external modules or tone generators.

The Motif is in an interesting grey area because it has rudimentary chord recognition, and "patterns" can be thought of as styles. The M3 and iPod are not arrangers, but one could say that arrangers are the next step up from the iPod, on a broad spectrum between recorded and live music (with phonograph LPs at one end, and acoustic instruments at the other).
My $.02,
Ted