There's a very interesting idea floating around. This idea says the upcoming PSP title, Metal Gear: Portable Ops is a little more than simply an MGS3 sequel -- it's a testing ground for new gameplay elements that may appear in Metal Gear Solid 4. Since MG:PO has some drastically different gameplay elements (4-man teams, limited inventory, recruiting any character seen to help you out, etc), Kojima and friends might be using the PSP game to see how people, and critics, react to these new styles of play.
You see, even though we've seen trailer after trailer of Metal Gear Solid 4, we've yet to see much concerning actual gameplay. An aging Snake taking on armies of mechs alone? Not likely. Many gameplay elements are completely unheard of -- this may be a reason why. What do you guys think? Is the PSP game a sort of test for how loyal enthusiasts react to gameplay changes that may appear in the PS3 title? It's an interesting idea, but at the same time, no one thought that Snake would battle with cards via Ac!d in future games.