I can just see all the "Hitchhiker" pagans out there scratching their collective heads. WTF does THE MEANING OF LIFE have to do with the number 42? Basically, this supercomputer chugs along for decades to figure out the meaning of life, and it finally calculates the answer as "42." It's an amusing gag. Not as life-changingly funny as some make it out to be, though.

(ducking the onslaught of hate mail)
JACKIE ROBINSON's number 42 has been retired by all MLB teams. Great inclusion for this puzzle.
PRESIDENT CLINTON was POTUS 42. I liked how 42 was indirectly referenced in the clue. Made you have to think for half a second.
I wonder how many people will write me today with confused emails asking what the theme was. Check out 42-Across: THEME. Get it? Three themers that all reference the number 42?
"But why is the meaning of life 42, Jeff?"
GAH I JUST EXPLAINED IT WEREN'T YOU LISTENING oh never mind.
This is a rare case where the concept might have been better as a Sunday grid. I say this so infrequently that I can't remember the last time it happened. But it'd be easy to make some sort of grid art, forming a 4 2 out of black squares. It wouldn't have had perfect symmetry, but that'd have been fine.
That would have opened up the possibility for other 42-related things. The 42nd element? The 42nd US state? Some neat mathematical property of the number 42? I'm sure there are dozens of cleverly indirect relations to 42, too.
Fantastic clue for DANTE. Maybe I should start writing about the underworld so that people would call me one hell of a writer too.
The clue for ITHACA confused me. It's ... Gorges? Apparently it has a lot of gorges. Maybe it's a snooty New York thing.
Fun idea to riff on the number 42. I wish there had been at least one other themer, and that the clues had more explicitly referenced 42 (so people would STOP ASKING ME WHAT THE THEME IS PLEASE STOP), but an entertaining idea.
ADDED NOTE: Constructor extraordinare Matt Gaffney astutely observed that it would have been perfect to run this puzzle exactly six weeks ago. I'll let you figure that one out.