I've seen one of those, too - in the middle of Utah!
I live in a major airline flight-path - all the airliners landing at KSLC pass overhead - and because Hill Air Force Base is only 40 miles north, we get a good amount of military traffic, too.
Generally, military traffic flies higher than civil traffic for the same reason.
One day, I heard a fairly quiet hum - sounded rather like the Robinson R-22s from my local airport - U42, just 3 miles west - but far more quiet - in fact, I almost did not register it as an airplane, thinking it was my cellphone vibrating - but the frequencies didn't quite match and I didn't feel any vibration, so like a true airplane obsessed kid, I looked up - and saw a V-22 (not an R-22!) almost straight overhead and flying lower than most of the civilian traffic to KSLC! The Huge rotor blades were spinning almost slow enough to count the blades, but that thing could really scoot along! In terms of angular velocity, its speed matched that of the landing CRJs we see heading toward KSLC ten miles north.
Me and my little brother had been sitting outside watching planes for about half-an-hour because he had seen a C-130 at Marching Band practice earlier in the day. C-130s are not a strange sight around my parts, but still cool, so we like to watch them - they have a certain rugged beauty that we Love to see, he and I. I did say the V-22 was quiet! My little brother did not hear it at all, and only looked up because I pointed it out to him.
VERY Lucky!
Paiken, was your experience similar to that? When I tell this story, I get strange looks from the ADolts who all thing the V-22 ought to be a very loud design - what sort of volume do you report?