I'll comment more tomorrow maybe but here is the link anyhow.
Hongbin Lin et al., Genetic Relationships of Ethnic Minorities in Southwest China Revealed by Microsatellite Markers. PLoS ONE 2010. Open access.
Something that seems pretty obvious is that Guandong Chinese (Han) cluster apart from Northern Chinese (Han) and together with SE populations (Tai-Kadai and Mon-Khmer). However there is no absolute divide and depending on the method of analysis used populations cluster somewhat differently.
The most divergent groups are Tajiks (no surprise here) and the Drung a small Tibeto-Burman nation from Yunnan.
Update (Oct 17 2012):
I never really got to review this paper properly but at the very least I must include here fig. 3:
... and fig. 4:
... which illustrate how Southern Han from Guangdong cluster with other SE Asians (specially Daic peoples) and not Northern Han (nor Tibeto-Burman, nor Mongol). Even more clearly than some other Southern ethnicities.