We're about to do it -well, about to do it in early October. We are finally going to hike from the North rim of the Grand Canyon down to Phantom Ranch, spend two nights, then hike back up the South. It is actually close to 20 miles, which isn't bad by itself except your basically climbing down a mountain than back up it again. As soon as we get our dates locked in I'll post them just in case there is anyone else out there who would like to join us.
And I know this isn't technically the "ultimate hike", that would fall to the British guy who walked the enter length of the Amazon river (and took two years to do it). Just watched that 2 hour documentary this past weekend.