In theory the game launches at 9pm (US) Pacific time, and Targets in my area are open until midnight. Anyone know if they'll be selling it tonight?

Even though you could get the game digitally before midnight in your area, I imagine the street date still counts physically as midnight in your area. The game "comes out" on 9/1/2015.

However, if you have no luck at Target I would try Walmart after midnight.

The game unlocks on Steam at 9pm Pacific. No telling when specific retailers are going to be selling the game though, you should call them and ask. Otherwise, there is already a thread dedicated to the MGSV launch:

http://www.giantbomb.com/metal-gear-solid-v-the-phantom-pain/3030-40796/forums/mgs-v-pre-install-waiting-room-what-time-is-it-1781533/#67