I needed to give the user an option to launch the application after install, and after googling for hours, I never really came up with a good approach for my situation.
That is, until I came upon Aaron Stebner's blog entry. This was exactly what I needed. Note: if that link ever fails, google for "How can I customize an MSI in the Visual Studio setup/deployment project?" and look for the hit on blogs.msdn.com)
Here's my post-build event contents:
cscript.exe "$(ProjectDir)EnableLaunchApplication.js" "$(BuiltOuputPath)"
There are no typos in that -- "Ouput" is needed instead of "Output".
If you have one Setup project that includes another, there is one caveat: you need to run the script on both your target dir (bin) and the build dir (obj) since project references pull components from the obj dir and not the bin dir at build time.