Almost every single package. (the ones that don't show that message are the ones that just hang the installer..)
Repaired permissions, grabbed installer.app from multiple installer disks and OS X installs, both Intel/universal and PPC, deleted prefs file, checked logs to see if there was anything that coincided with the error, and so on. Don't remember when it started, or why.
Don't want to reinstall just yet, so if anyone has any pointers or stuff to try, ...that would be awesome.