This is a sketchy workaround, as it could be susceptible to issues caused by data irregularities; GSW notes, "What happens if you don't have your pieces of data correctly sync-ed and one of them is an old version of an object/piece of code and one is a newer version?" A better, albeit unpopular solution, is to include an option to install a chunk of a game directly onto the hard drive (Genji has this option; so does Ridge Racer). Bethesda's use of the extra Blu-ray disc capacity is clever, but hopefully won't become the standard.
Overall, we know that the Xbox 360 drive reads data faster than PS3's. While the Blu-ray drive might consistently read data at the same speed, the 360 drive is faster at reading data on the outer layers of the DVD disc; and that's were devs tend to put the biggest files.
While EGM makes no note of it, it's worth considering that drive speed concerns played into Oblivion's delay. Will this become an issue for other cross-platform titles?