Personally from what Dr. Breen is always going on about, evolution and such, i think those white creatures were humans. They've been altered somehow in some sick twisted way to become "trans-humans". They may very well be the same thing that is inside those combine uniforms. After all, breen calls them the "trans-human forces" in nova prospekt.
Also, in some parts of the game you can see a painting on walls that depicts a monkey evolving into a human, and a human evolving into a combine soldier. This means that Breen's and whoever that big white blob thing he was talking to in that screen before he attempts to make his getaway are intent on converting humans into those...things...whatever they were. The process may very well be why they are sucking the planet dry of resources. I guess we'll find out how this myserious force Breen answers to reacts in hl3. Personally i'd like to see valve fit our good buddy Corpral Shepard into all this too, that'd be awesome.
DAMN i love this game, it makes you think so much every time you uncover some little detail!!