I predict that the 720 will have a dual SKU situation with the following:
1.) Arcade-like unit: DL only to save manu. costs. Most casuals are not big DLs anyway. I could see a USB drive with XB/X360 compatibilty as an "upgrade" accessory.
2.) Elite-like unit: Flash-based HDD with a proprietary Microsoft high def DVD format. This puts them at an advantage by avoiding the Blu-ray forum liscensing costs.
With this dual SKU 720, MSFT can reduce confusion a cost for the Arcade potentially gaining more casuals. The Elite will be future proof and highly expandable like the PS3.