Blank screen on install, 500 error at set_include_path()
This is one of the strangest Civi911 calls we've gotten yet.  The problem definitely was set_include_path(), but it wasn't consistent with a more common problem with that--that it doesn't take effect.

We actually spent the maximum amount of time and handed it back to the client, telling him that we narrowed it down to set_include_path() but didn't know anything more, and we suggested he ask his host about it.

The host said that they have installed the cPanel/WHM Magic User Loader, which allows a user to have its own ~/php directory for PEAR modules installed via cPanel.  Enabling the Magic User Loader causes set_include_path() to fail (miserably, as we saw).  The host disabled the Magic User Loader, and the client ran the installer with no further problems.