Why would they announce a bigger HDD for recording TV when the service is nearly 9 months away? Good way to gut sales. Same reason no SKU differences or price drops were announced. You want to clear current inventory and keep current sales strong until you make such a move.
Because they can introduce an optional bigger HDD without waiting for IPTV or revising the base model ? Some already want a bigger HDD for the current movie and TV download today.
I also thought they were going to release the IPTV soon (together with content partners and distribution relationships). I now wonder if we will see other announcements between now and Fall 2007 then (like Apple's long rumored MacTV in MacWorld).
As for clearing old inventory, it is also not uncommon for vendors to introduce "one-off" discounts/promotions (like you see in the holidays).