PHL have just posted an exclusive look at Siggraph 2004, at which Valve's art-duke Viktor Antonov took part.
http://www.planethalflife.com/features/articles/sigg04/
Some quality new screens there.
EDIT: Seems someone's posted a thread on this at the same time as me. Ah well, more the...
Sorry to rain on your parade, but Deus Ex was linear too. Although you are given multiple choices in places, you are still progressing forward by completing tasks from a start to a finish. Really, no game is completely non-linear, but I suppose the GTA series is quite close to such a fabled and...
From PC GAMER UK:
"Other Half-Life 2 environments that Valve have revealed to us include a massive prison complex, which from interior and exterior shots seems to be modelled to some extent on Alcatraz."
While this doesn't confirm it isn't Alcatraz, I believe it definitely is not said...