I've been playing with soil moisture sensors but I may not be much help.
I originally set out to use homemade gypsum block sensors such as these: Untitled The problem I had was that when I tested them the soil would get bone dry before the sensor reading would start to drop. The plaster just seems to retain moisture far too long to be useful.
I created a homemade version of this sensor instead: http://www.hobby-boards.com/catalog/product_info.php?cPath=22&products_id=1547
It seemed to work well in indoor tests I did last winter but since I put them out in my garden a month ago we have yet to go more than 5 days without rain so the soil really hasn't gotten dry enough for a real test.
I'm using a 555 timer circuit to measure conductivity through the sensor.
Here is a full description of the circuit.
Rather than using the voltage across a resistor to measure the current to the timer circuit I just connected the discharge pin (pin 7) of the 555 to one of the Arduino interrupts and count the frequency which increases with moisture level.
I'm not doing anything with wireless. I just have a red and green LED. Green if the frequency is above a certain value, red if it is below.
As I said, if it ever stops raining I'll let you know if it works. 