Well, I don’t know about most. But I don’t know anyone in my corner of California who doesn’t pronounce those the same. Harry, hairy, merry, mary, marry - all identical to my ear.
Well, that’s just it, though. I can’t think of anyone I know who doesn’t pronounce those the same. You tend to be surrounded by people who speak with the same general dialect that you do, so it’s hard to make generalizations one way or the other just based on the people you know. (This goes for the generalization that most Americans do pronounce them the same too, of course.)
My accent is quite distinct from those in my general area, which is probably why I’m ultra sensitive to dialects, pronunciations, etc… It fascinates me when people consider words like, say, ‘pen’ and ‘pin’ to be homonyms, or ‘caught’ and ‘cot’.
I pronounce them *almost *identically though I know exactly what the sound difference is when people from other regions prounounce it, specifically American East Coast natives. My first name is "Shari’ and I’ve had people, when getting spelling clarification, say “oh it’s SHAAA-ree”. Um, not really but who am I to quibble about someone’s native accent.
Interesting, from Wikipedia:
Mary-marry-merry merger
One of the best-known pre-rhotic mergers is known as the Mary-marry-merry merger,[4] which consists of the mergers before intervocalic /r/ of /æ/ and /ɛ/ with historical /eɪ/.[5] This merger is quite widespread in North America.[sample 1] A merger of Mary and merry, while keeping marry distinct, is found in the South and as far north as Baltimore, Maryland, and Wilmington, Delaware; it is also found among Anglophones in Montreal.[6] In the Philadelphia accent the three-way contrast is preserved, but merry tends to be merged with Murray; likewise ferry can be a homophone of furry. (See furry-ferry merger below.) The three are kept distinct outside of North America, as well as in the accents of Philadelphia, New York City, Boston, and Providence, Rhode Island.[7][sample 2] There is plenty of variance in the distribution of the merger, with expatriate communities of these speakers being formed all over the country.
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IMHO, widespread=most 
I know I’m being pedantic but I’m sure you’re aware that the word ‘widespread’ has less to do with volume than area, and does not necessarily indicate a plurality or majority.
Furry-ferry merger, eh? I was not informed this was going to be one of those threads.
As long as you know you are being pedantic, and not just about the meaning of widespread. 
The m/m/m merger is out there, whether you like it or not.
This only makes sense if you personally know most Americans.
How big is your Facebook Friend list, anyhow?! :eek:
They do in my dialect. If you look at the threads I linked earlier, you’ll find links to actual recordings of actual Dopers pronouncing these actual words. Some make them rhyme; some don’t. (And the one that sounds like Kermit the Frog, that’s me.)
I dunno. I live in San Fran also, and know nobody who pronounces any of these similarly. I’ll give you Merry / Mary Christmas, but not Merry / Marry Christmas. Hairy / Harry? What’s on your head, hair or harr?
Joe
You say Harr-ee where you live? If I try, I can manage a slight difference between hairy and Harry, but it is darn slight. When I don’t try, they sound exactly the same–hair-ee.
Hairy sounds like hare, as in rabbit. Hairy, fairy, Mary, berry, Terry, Jerry. Harry sounds like marry, carry, Gary, Larry, Barry…
Joe
I know that example works for you, but marry, carry, Gary, Larry, Barry Hairy, fairy, Mary, berry, Terry, and Jerry all rhyme to me.
Me, too.
Farrell rhymes with carol. Ferrell rhymes with peril.
but… but…but… carol and peril rhyme to me.
Farrell and Ferrell have a slight difference to me, but it may be only discernible to other Mid-westerners. It essentially boils down to whether or not I’m “smiling” when I say it.
When I lived in Massachusetts, I was endlessly confused by how the natives pronounced Don and Dawn the same. They’re nothing alike!
I finally heard this! Last night on some sitcom…“How I Met Your Mother”, maybe? This girl kept saying “dawn” when talking about her ex, “Don”. Drove me batty!
They’re not likely to be the same people though, right? Pen and Pin are distinctly different here, but even though people tried to explain in another thread, I can’t wrap my head around caught/cot and taught/tot not being homonyms. (I think we did narrow it down to us using the “aught” for both and never what others say for the “ot” words). Someone even offered a sound file, and it was hard to accept the version we don’t use as someone not just exaggerating for effect or realize without context which they were saying.