Amazon has just added the ability for smart home skills to recognize contact and motion sensors. So far the only sensors which work are those connected to SmartThings (yay!) Or ones that can connect directly to the echo plus zigbee coordinator.
You will be able to use the sensors to trigger echo routines (not smart things routines, although you could do that indirectly by turning on a virtual switch) and you will be able to query their current state.
Among other things, this should mean that we see the price of zig bee contact and motion sensors come down as manufacturers go for the echo market.
Updated @nezmo and I literally started new threads in the same minute. LOL!