I've always heard a lot of Japanese people pronounce ありがとう as ありがとう{LHHHH}, but I've never actually seen anyone talk about it on the Internet.
People always say ありがとう should be pronounced ありがとう{LHLLLL} in standard speech, which obviously I hear a lot, but I also hear a lot of speakers of Standard Japanese pronounce it as an unaccented word, so I definitely think it's not dialectal. (I even hear ありがとう{LHHLL} a lot of the time).
Is it pronounced unaccented because it's a shortening of the phrase ありがとうございます{LHHHHHHHHLL}?
As far as I'm concerned, the phrase can be pronounced two ways: with the downstep on ま, or with a downstep on り and then on ま (ありがとうございます{LHLLLLHHHL}).