It's possible that there's a way to do this with a Mac application, but the only way I know to change this setting is from the command line in the Terminal application (Applications --> Utilities --> Terminal.app).

Open it up and type in the following command:

$ pmset -g
Active Profiles:
Battery Power           -1*
AC Power                -1
Currently in use:
 sms            1
 hibernatefile  /var/vm/sleepimage
 acwake         0
 sleep          30
 halfdim        1
 hibernatemode  3
 disksleep      10
 displaysleep   10
 lidwake        1
 lessbright     1

The key setting there is lidwake which is enabled, as you can see by its "1" value. To disable this particular setting you need to use the "pmset" command again, but this time you need to be the administrator since you're actually changing something:

$ sudo pmset -a lidwake 0
$

Now if you run the "pmset -g" command again you'll see that the "lidwake" setting is zero, not one. That's all you need to do. Now, until you use the command to reset the value to 1, you'll have a Mac that doesn't wake up when you open the lid.

By the way, while we're looking at the "pmset" command, note that there are a number of other settings you can tweak, including the time-to-sleep, whether your screen should dim, and a lot more. Check out the man page for the command (type "man pmset") and scroll down to the bottom to read all the examples. Very useful utility!

By the way, my PowerBook had the same problem with a flaky screen latch, a problem worsened by the screen warping. I finally ended up always leaving it screen-down and having a tightly-fitting fabric envelope to keep the unit closed in my computer bag. It would have been a far smarter thing to have known about "pmset", however!

Good luck to you!