I have known this for quite some time.
In an Interview here is what was said about Metal Gear Acid...
Tips & Tricks: Can you comment a bit about Metal Gear Acid for the new PSP system from Sony; maybe explain how this game fits into the over all series?
Hideo Kojima: As for me, it's about three years after Metal Gear Solid 2. My feeling is that the names Metal Gear Solid and then Metal Gear Acid, so they should be considered as completely different games.
Personally I don't know enough about it right now to comment on how i think it will turn out. I haven't been really following it.