I liked the idea of a PT100 reader with 0.03 C resolution, and Adafruit already made the library available, so I quickly hacked support into Marlin if anyone is interested (I think there's currently a feature freeze, so I'll submit it later). Note that this only works for heater 0 (it piggybacks off MAX6675 support).
1. Get the Adafruit MAX31865 breakout board
2. Wire the board.
On RAMPS 1.4, Aux 2 top row = GND, 63[sdi], 40 [sck], 42 [sdo], 65 [cs]
If you want to change pin assignments, look for this line in temperature.cpp