On a GNOME 2.x desktop if you are a laptop owner you can control what happens when you close your laptop's lid from the power management preferences whether to suspend the system or simply blank the screen. With GNOME 3.0, when you close your laptop's lid, the system will suspend and there will be no user-interface for changing this policy. It's a design decision for the GNOME 3.0 desktop.
In a blog post entitled Is GNOME 3 going to melt your laptop?
, Richard Hughes of GNOME Power Manager and UPower wrote "In GNOME 3.0, we’re defaulting to suspending the computer when the user shuts the lid, and not providing any preferences combobox to change this. This is what the UI designers for GNOME 3.0 want, and is probably a step in the right direction. We really can’t keep working around bugs in the kernel with extra UI controls."
If your laptop lid is closed but there is an external display attached, the system should not suspend, except for black-listed notebooks. This though cannot be configured from any user-interface setting. However, it looks like there may at least be a dconf / GSettings key that can at least be manipulated to change this behavior.
Since writing about this GNOME power management change yesterday, there's already more than 140 comments of prospective GNOME 3.0 users that are mostly objections to this move. There are a number of use-cases where one wants to close their notebook but to have their system still powered up -- downloading files, wish to keep their online presence, temporarily going away from the keyboard for a few minutes, etc. I am in the same boat as many of those users. More importantly, for still a lot of hardware out there, suspend support under Linux is still quite buggy or completely broken.