I'm late with this, but you may have missed it. In an interview with Famitsu magazine, father of Metal Gear, Hideo Kojima, has apparently claimed that Blu-ray does not offer sufficent storage capacity for MGS4.
"There's not enough space. We always talked about where to cut and what to compress," he laughed, impressing an internet full of gigabyte geeks who think, "hey it must be amazing if it's so big it won't fit on a 50GB dual-layer Blu-ray disc!" and not, the more accurate, "oh dear god, that means days of cut-scenes!" or the less charitable, "oh shut up and just release the damn thing."
Data bloat does not necessarily equal quality.
MGS4 will be hitting the shelves, complete with its beta of Metal Gear Online, on June 12.