The 2012 Olympic Games' London Opening Ceremony started at 4 p.m. EST, but for some reason NBC decided to delay the broadcast by over three hours. (NBC is streaming every event besides the opening ceremonies, which pretty much everyone cares about. They haven't explained why. Go figure.) You'll be able to watch them starting at 7:30 p.m. EST.
HOWEVER. Lots of people are already watching in real time, and according to my Twitter feed, shit's pretty crazy. I've been too busy to watch, but apparently (SPOILER ALERT) there's some sort of free health care dance celebration?