In addition to that there were incidences of untouchability in Nepal and Japan that appear to have a common factor in Buddhism.
But I have come across many studies that point out that many professions were considered low and hence the people in many societies. In other societies it was just a social phenomenon but in India it was religiously sanctioned feature.
With this background, I feel, there exists ambiguity about Buddhist attitude towards untouchability. The problem is compounded as the development of Untouchability that included slave farmers may be post Buddha. When it comes to untouchables who were slave farmers*, I think that it has something to do with Vaisyas(free common men like farmers, herders, smiths) becoming Sudras thus further pushing Sudras(slaves/serfs) into Asprishyas(who probably during Vedic period included undertakers, forest tribes etc... only).
Recently I came across this message at Indo-Eurasian_research Yahoo group. It's about Untouchability in historical China.
"Untouchables" long existed in China in the form of various groups of jianmin
"lowly people". They were clearly separated from regular, "good households"
(liangmin) in official population registers. Their exact components varied from
place to place. But most were in such universal "untouchable" professions as
entertainers, undertakers, prostitutes, professional beggars, garbage
collectors, etc. Some groups had fancy stories about their origin not unlike
that of Roma people/Gypsies (re. the nails to be used to crucify Jesus). Those
lowly people were forbidden to marry with "good households". Nor were they
allowed to go to school and to participate in civil service examinations.
From 1723 on, Emperor Yongzheng (reign 1723-1734) issued repeated edicts
abolishing such "lowly households" in government population registers. Two
particular remarks can be made here:
1. It took a "Barbarian" Manchu emperor to legally abolish "untouchability" that
had existed in China for centuries.
2. Emperor Yongzheng's decision was inspired by the Buddhist maxim that "all
lives were created equal," a notion that came from the Indian subcontinent among
Naturally, Qing legal actions could not eliminate many or perhaps most of those
"lowly people" who continued to engage in their traditional professions. For
instance, in Shaoxing (Zhejiang province), even on the eve of the Cultural
Revolution, many undertakers still came from traditional "duomin" families.
A somewhat meek point is that, because China's "lowly people" almost never
engaged in farming (which in Confucian ideology was the second most respectful
profession), untouchability in premodern China represented a very small and
mostly urban social component.
Buddhism in the hands of nomadic people (who didn't show much interest in purity-pollution) probably showed its true face at least in one aspect.
Note: Farming might have been noble profession in China but not so according to purity rules of Hinduism. Though it wasn't as low as the so-called low professions like scavenging, leather works etc... it wasn't an exulted profession. However, it was never deemed too low as almost all sections of the castes engaged in it.