Southern Thai (also known as Dtai or Pak Dtai, hence it “counting” as the language of the week starting with D) is one of the three most widely spoken varieties of Thai. (My information for this post is mostly coming from this overview of the linguistic situation of Tai languages.) Thai itself is a member of the south-western branch of the Tai family, a language family which also includes Lao and Ahom.
Southern Thai is spoken, as you might guess, mostly in the southern regions of Thailand and is in extensive contact with Malay. It appears that many of the differences between Southern Thai and Standard Thai are due to borrowings from Malay (although I’m not sure how to reconcile this with other information, such as Southern Thai having more tones than Standard Thai – it’s hard to see how that is influence from Malay, when Malay is not a tone language).
The picture of the dialectology given in the Ethnologue is a bit different – it would appear to place Southern Thai as its own subgroup of Tai. Perhaps this represents confusion with Souther-western Tai (one of the three subgroups of the Tai family, to which Thai and Lao belong).
There’s a (pdf) report on a linguistic survey of Thailand here. It’s marked “commerical – in confidence”, in which case it might disappear. It seems to be aimed at working out the market potential for Thai speech recognition systems, and the linguistics is sort of basic…
The linguistics department at Chulalungkorn University has a nice Thai language resources page, including a list of Theses completed in the department.