I'm not sure what else to suggest ... assuming you have the correct F-7-ppc-DVD.iso file, it checks out OK with sha1sum, you're burning the DVD with imgburn, verify on, no errors reported, you have the correct bootloader as linked in the
first post of this guide, and the install DVD passes the media check ... also your PS3 firmware should be up to date (2.20) ...
Normally this isn't a problem, but make sure you don't have any other USB devices or memory cards inserted when installing -- just the mouse and keyboard.
Other than those suggestions, I'm running out of ideas here...