Oh finally, this thread was made. Now I can post my all-to-long opinion that very few people care about!
I like to think that each game in the Orange Box takes place in the same universe, just at different times. TF Industries started around....whenever Zeph founded it, then along came Aperture in the 1950's. Aperture went bankrupt in the 70's, and were bailed out by TF Industries or whoever. Then a bunch of stuff happened up to the events of HL2, and the Portal games.
Something in my gut tells me that they're not alone in that universe; that there's more games in the years between 1975 and 20--. A little part of me likes to believe that this all takes place on the same Earth that Lord of the Rings took place on eons ago, mainly becuase of the magic in HL2, and other things that cannot be explained with logic. But that's just my dumb and poorly-conveyed opinion. Have a great day!