Yes, there can be battery drain because of constant network activity. Probably, android client should strictly limit number of outbound connections, but I think it's acceptable to network because of potentially gigantic number of devices.
this could make Nxt a truly mobile payment/messaging/e-commerce platform in the palm of your hand...extremely user friendly...if we can pull this off it would drive adoption thru the roof.
Updates could be pushed out automatically or thru the Apps store to each device which could potentially number in the millions.
EDIT: Anonymous mobile payment/messaging/global e-commerce platform in the palm of your hand.