According to [Farnell's Quick Start Guide][1], the default baud rate is: **115200**. A fairly detailed discussion of the UART settings and capabilities can be found in this blog post [More on Raspberry Pi serial ports][2]. More specific implementation details/challenges of working with the serial port can be found in the blog post [Getting my Raspberry Pi set up for high-speed serial UART communication][3]. Finally a good discussion on setting the UART's baud rate can be found in Andre Miller's article [Raspberry Pi and Arduino via GPIO UART][4]. 

Remember to watch your pin voltages to avoid damage to you Pi.


  [1]: http://www.farnell.com/datasheets/1524403.pdf
  [2]: http://lavalink.com/2012/04/more-on-raspberry-pi-serial-ports/
  [3]: http://fw.hardijzer.nl/?p=138
  [4]: http://www.andremiller.net/content/raspberry-pi-and-arduino-via-gpio-uart