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]. 

You don't mention which distribution you are running, but assuming you are running Debian you can set the baud rate by doing the following:

Note the Raspberry Pi uses the UART for Console Messages (including bootup messages) and getty so you can login via serial. To use this serial port for your own uses you will need to disable these services. 

To change the console baudrate, edit **/boot/cmdline.txt** relacing **115200** with your desired baud rate (note this is all one line).

> dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200
> console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait

then edit **/etc/inittab** to change the baudrate of the getty (you
should find a line like the following with the baudrate of 115200, change that
to your desired baud rate)

> 2:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

and, remember to watch your pin voltages to avoid damage to you Pi.

Additional References:

http://www.andremiller.net/content/raspberry-pi-and-arduino-via-gpio-uart


  [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