I have not planned to add soil moisture monitoring to OpenSprinkler yet. This is mainly because I suspect most users still prefer a time-based program, with potentially some weather-based refinement. Adding soil moisture sensor will potentially complicate the scheduling, and in case the sensor fails to return correct reading it can be come a source of reliability issues.
I do have an upcoming product called OpenSprinkler Sip, which is designed for indoor plant watering and it does use a soil moisture sensor. You can take a look at the picture on this webpage:
http://rayshobby.net/