I think we need to keep in mind that Sony was trying to build this thing to a price point -- they wanted the 300 to be the first eInk based reader for under $200. Card readers, and an mp3 player may not have cost much, but I bet they would have added $20-$40 to the cost of the unit. Either Sony would have had to accept less profit (or maybe virtually no profit) or raised the price on the unit.
The Jetbook has several advantages on price:
1. A reasonably conventional LCD screen.
2. No licensed software.
3. Plastic construction.
Now mind you, I am not saying the Jetbook is cheap... far from it, I have found it to be rather durable and it is an excellent machine. But I think the Sony Reader needs to be taken on its own merits.