You have to run an installer to install SketchUp, and even
though the icon looks like a standard Mac OS X installer
package, it’s actually a crummy installer from MindVision. It
does not tell you what it’s going to install, or where, or why
it requires you to authenticate. Looking at the installer log,
it’s pretty innocuous: it puts the app in /Applications/ and a
bunch of support files in /Library/Application Support/Google
SketchUp/. I moved the app to ~/Applications/ and the support
folder to ~/Library/, and the app still seems to work properly.
Any installer that doesn’t offer you a list of everything it’s
going to install sucks. The worst part, though, is that this app
shouldn’t need an installer in the first place.