While there is a PS3 bundle planned for Metal Gear Solid 4: Guns of the Patriots, there will definitely be no demo prior to the games release.
Ryan Payton, the host of the latest Kojima Productions podcast, made the announcement: “I can tell you, as of right now, we don’t have any plans of uploading this demo on the PlayStation Store. And definitely no plans of uploading on the Playstation Network next week.”
Oh well. MGS4 along with Final Fantasy XIII is expected to be the biggest system sellers for Sony, so they don’t really need demos to convince people to buy these titles.