how to boot a from a DVD? On a friends older MacBook, I'm holding the ⌥ Option key with a bootable DVD inserted but am not seeing a boot menu.  I've tried rEFIt as well as some Linux DVD's I have lying about.  I've checked the DVD's on another system and they are bootable on non-Mac hardware fine.

In excruciating detail, what's the process for booting from a DVD for the Mac?  I've tried holding the ⌥ Option key, tapping, holding until I see an Apple logo -- and everything in between.

I've previously gotten this particular machine to boot live Linux DVD's.  As I recall, that was with the option key -- and, I don't recall having to hold and release at specific junctures.  It has since had a Mac OS 10.x clean install.
I installed Bodhi Linux, and then Lubuntu.  I don't recall jumping through any particular hoops, beyond, as best I recall, holding the option key until getting boot options.  It's like a firmware update, or perhaps newer OS, somehow made it harder to boot Linux or Linux type discs...?

As Allan suggests, the Genius Bar likely restored back to factory.  Which doesn't explain why option worked to boot to DVD before but doesn't anymore.
Press and hold the C key on boot to attempt booting from the optical drive.

If that doesn't work I'd recommend trying the boot CD in another machine, and a different boot CD in the mac.