At this point the Metal Gear franchise is diverging into a new entity, while Peacewalker contributes to the 'classic' entity. This is a great move as it allows Metal Gear to expand and benefit from a switch up of gameplay designs that were otherwise very limited in the Solid series. That isn't to say that the new divergence should not be labeled a 'true sequel' in terms of additions to the universe, MG timeline, but what I see Peacewalker as is a sequel to the spirit of the original series. Resident Evil did the same thing when they altered the camera angle and allowed third person aiming/control, while still making games that used the fixed camera system. It made for very different games that were bound by the same universe (or at least tried to - RE5 kinda let the firecracker explode in its own hands).

I do think it's a shame that the final word on the Solid series is being restricted to the PSP. I think that much could have been edited out of the wacky and nonsensical MGS4 that could have covered a majority of what needed to be covered, and then the PSP release could elaborate further.