Mountain Lion install can't be verified, is damaged or tampered with?

I'm trying to install Mountain Lion on a mid-2009, 13-inch MacBook Pro. When I run the installer, I'm told it can't be verified and may be damaged or was tampered with. The error message urges me to delete the file and redownload it. I've tried that, and still get the message.

I've successfull installed Mountain Lion on my mid-2007 iMac, and tried copying the installer file over to the MBP. When I do, I get an error message saying to run the file on an Extended Journaled partition, which of course is what the MBP's hard drive is. The drive is a WD Momentus XT 750GB drive. It passes all verification tests and yes, I've repaired permissions.

As I write this, I'm trying a third download of the installer. Any thoughts?