gistfile1.rb

require 'rubygems'
require 'sinatra'
require 'serialport'

port_str = "/dev/tty.usbmodemfa131"
baud_rate = 9600
data_bits = 8
stop_bits = 1
parity = SerialPort::NONE

set sp: SerialPort.new(port_str, baud_rate, data_bits, stop_bits, parity)

post '/' do
  settings.sp.putc("a\r\n")
  'Success!'
end

Care to explain what this does? Any setup?

The only set up is probably changing port_str to the name of the USB device on your box. The rest of the params are pretty standard (and the baud rate of 9600 is specified in the Arduino part as well), and the only writing going on in this case is the #putc method. Also, Arduino seems to only read a string once it gets a CRLF (\r\n).