I'm speculating an internal motherboard revision of the slim, maybe an even cheaper blu-ray drive, maybe even 45nm RSX if they can manage it, to reduce costs, so they can include the motion controller with the $299 unit, and sell a "core" unit for $249 (or less) without motion control, and they couldn't hope to make those changes in time for Spring yet, but possibly for Fall. Maybe even the elusive rear USB port (I just use a 7 port hub and tuck the cable to the rear)
There was a rumor from an Asian source early last year that Sony will release another SKU this year (i.e., after Slim). We shall see.