Since Pacifist allows you to see exactly what a package will install and where, you can also examine the install scripts if you click on the Resources tab. So if a program were going to stick something unsavory in /Library -> StartupItems or /System -> Library -> Extensions, you would find out about it before the installation.
So if you've downloaded a package and you'd like to know what it's going to do before it does it, take a look at it with Pacifist first. But now I wonder if someone could write a script or such, so that Pacifist would automatically launch every time a package were double clicked. Waddya think?
[robg adds: Somewhat obviously, this only works for programs that use the package installer.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060406112833259