Only 2 USB ports seems to imply a new form factor.
Just going from 4 to 2 ports using the same housing doesnt seem to make much sense. The cost saved would be so insignificant.
A new form factor this early in the life cycle seems unlikely to me, the original 20 GB unit without the HDMI port had the same formfactor as the 60 GB, in a similar way this will just be a cheaper version with some features removed. If you buy a PS3 with just 2 USB ports you know you will have to get a hub the day you want to attach more than two USB units at the same time. For the majority of users 2 USB ports will be just fine.
There is also no point in trying to match the features and prices of the rumored units to the BOM as some people try to do. The value will be in the eyes of the buyer. The cheaper unit will likely be more subsidized than the more expensive unit, but the buyer who has decided to buy a PS3 really doesn´t care about that, if he/she wants 4 USB ports, larger HDD and has the money, he/she will buy the more expensive unit. Perceived value bla bla....
@-tkf-, I think you will already find all ports on the main board.
http://pc.watch.impress.co.jp/docs/2006/1111/ps3_32.jpg