I would guess it went down because of the HTC phone and that could be why walmart started selling it edit: (they were not previously been selling it have they if they were I was not aware that they were.) because now they could get a super deal on an inventory surplus.
My guess is they will go all out selling it at that price until the HTC one v comes out then it will take another dip. So if you want a cheap mt wait for HTC or if you want the latest and greatest also wait for the HTC . The second dip won't be much though. I've worked at walmart as my second job for the last 4 years and have noticed how they operate they love surplus .