well, my understanding was that portal 2 is only set a few years after the original portal, with Chell being held in stasis again. That should negate the need for time machines. I'm fairly certain that it will come out on the 19th as planned (due to all the TV spots, videos on the teaser site, and demos at conventions), but I might be biased because I preordered.
also, you know what WOULD be like valve?
just making this achievement a code hidden in a box in a secret level, and the code is almost the same color as the wall, and it's in @#$&ing greek, so all of this will just stay as it is: a bunch of wild extrapolation on internet forums by people with too much time on their hands (like the ones who decompiled the aperture science website).
(disclaimer: I have no problem with this sort of thing, and admit to partaking in it)
Oh yeah:
and I see 2 achievements that DEFINITELY involve the companion cube (one in singleplayer, one in co-op)