There is heightened security all over Beijing. The local news includes features about how this is an effort to reduce the risk of terrorism.
It's nowhere near the level of security we experienced in New York after the 2001 terrorist attack but it's still interesting to observe and experience.
There are bag checks in the subway. Depending on the station and the staff my experience has included walking right past the security line and not being bothered, having a large bag x-rayed as i was headed out of town for the weekend - and being asked to take a sip of my water to prove that it's not an explosive, and sticking my bag on the x-ray machine conveyor belt and calling to the tech so that she'd look at the bag (I was in a hurry and didn't want to play games, or to have to do it again).
Getting into some Olympics-related parties requires a passport. Others also require that your name be on an approved list. I'd been leaving my passport at home recently but am now carrying it again just to be sure I won't have a problem.