Does anybody know if Dead Space 2 will be unlocking tonight at midnight or tomorrow morning?

Not certain, though based on past releases we will be lucky if it unlocks tomorrow.  It seems that the most anticipated games are always the ones that have launch issues on steam.  I wouldn't stay up unless you have nothing better to do though.  Hate to sound negative, I love steam, but this is just the safest approach after all the lackluster launch days I had spent just praying the game will unlock at some point. 
 
Also, the lack of a countdown timer does not inspire confidence.

One can hope for midnight, but most likely around 10 AM PST tomorrow.

Mid Day is when I notice the real changes to the storefront but they might be prepping the servers for it tonight.  I'd bet on mid day tomorrow.

usually 10am

Tomorrow 1000 PST. That way if something goes wrong they have people to fix it.

My game is unlocked as of this post. Go to Steam > Check For Steam Updates if yours still says preload.

This game will unlock in approximately 1 day and 15 hours
 
For me in the Netherlands at least :(