Hi Chris,
Not so far. In order to have them at under constant distance, i used an old wall plug connector. So the terminals works such as the nails... They could not go so deep, but it could be used like an "standard".
Take a look to what i did here:
http://ardudrop.jottit.com
In any case, you decide the distance, because it require a calibration by the 10K pot and also by code... Select you distance, and calibrate it to 0% of water and 100% of water in the soil.
I hope it could be useful for you.
Cheers,