Hi Brambel, Anne,
I live in Bali and work in Sumba as my partners and I buy and sell a lot of lands there.
Anne is right in a way, tourism doesn't really show yet or is not obvious like Flores for instance.
However hotel capacity has more than doubled in Tambolaka. Mostly businessmen go there though.
We sold a many plots to foreign as well as local investors and quite a few of them are planning to start building in 2016, including ourselves.
Infrastructures (roads, airport, etc...) are getting better and better (unlike Flores where nothing has changed in the last 10 years) and it looks like Sumba is getting ready for the tourism wave coming its way.
Go check out our website, read the blog post and our brochure (free download), you'll find a lot of up to date infos on Sumba: http://www.01islands.com
Feel free to contact me if you need more infos.