How to read 115200 Baud digital signal on Arduino UNO?
SoftwareSerial only works up to 38400 bps, because running faster, it overloads the computation capability of the CPU. The documentation for SoftwareSerial on the arduino.cc site mentions this.

To read a 115200 kbps signal, you have to use the built-in serial port. If you also use that for programming, you're going to have to do some connecting/disconnecting each time you want to program your part -- or you can get an In-Circuit Serial Programmer and use that rather than the serial port for uploading. I recommend a Avr-ISP-2, which can be had for $35 at digikey. Ignore the CD that comes with it if you're using the Arduino IDE.

In general, a better source of support for Arduino-specific questions is the forums at arduino.cc.