The latest issue of PlayStation: The Official Magazine has more info on what-looks-like playable Metal Gear Solid memories in MGS4. According to the magazine, cut scenes at the beginning of the missions refer to the franchise's evolution. What's more:

...as characters talk and reference other stars of this universe, you may see an X or L1 button icon quickly flash on the screen. Hit the right button while it's highlighted and you'll be treated to a very brief context flashback. Basically, the system uses images and moments from the past Metal Gear games to help visually rekindle your own memories of the who, what, where, and why.

Thanks, T.G. for the tip!

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