I just recently purchased the battery powered Yamaha QY70 ($405 at Guitar Center including AC Adapter, headphones & 2 yr. extended warranty) and found out that in addition to being a fantastic ultra-small (7-3/8" x 4-1/8" x 1-11/16" ; 1 lb - 2 oz) yet "full featured" pro quality sequencer & XG sound module that it can also act as an arranger keyboard in that you can midi it up to an external battery powered keyboard controller and play in "arranger keyboard style" split mode (playing chords in the left-hand triggering the QY70 to play auto-accompaniment patterns, while you play the melody in the right hand).
There are a few arranger feature limitations (such as notes played left of split point 'fingered zone' do not sound and no sycro start button). Even so, the QY70 is the smallest & lightest arranger keyboard alternative available anywhere and is well suited for taking and practicing on the road (car, airplane, hotel rooms, etc). The sound quality and features packed in such a tiny package are truly awesome.
Many thanks to Clif Anderson, George Kaye, Uncle Dave, and Tom Griglock for their valuable input.