As I couldn't find any decent sample code for reading and writing through the serial port in Mono/.NET, I decided to write my own. In the end it's fairly simple, and should work on both Mono and .NET platforms (the code is fairly self-explanatory).

Reading from serial

// SerialTest.cs
//
// The MIT License
//
// Copyright (c) 2008 Matias Korhonen
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
using System;
using System.IO.Ports;

class MainClass
{	
	public static void Main(string[] args)
	{
		SerialPort sport = new SerialPort("/dev/ttyUSB0", 9600);

		if (sport != null)
		{
			if (sport.IsOpen)
				{
					sport.Close();
				}
		}

		Console.WriteLine("Arduino Serial Test");

		sport.Open();

		while(true)
		{		
			Console.WriteLine(sport.ReadLine());
		}
	}

}

Writing to serial

// SerialWriteTest.cs
//
// The MIT License
// 
// Copyright (c) 2008 Matias Korhonen
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
using System;
using System.IO.Ports;

public class SerialWriteTest
{

	public static void Main(string[] args)
	{
		SerialPort sport = new SerialPort("/dev/ttyUSB0", 9600);

		if (sport != null)
		{
		if (sport.IsOpen)
			{
				sport.Close();
			}
		}

		Console.WriteLine("Arduino Serial Write Test");

		sport.Open();

		String inputString = "";
		Console.WriteLine("Toggle LED (write 'e' to exit)");

		while(!inputString.Equals("e", StringComparison.CurrentCultureIgnoreCase))
		{		
			inputString = Console.ReadLine();
			sport.WriteLine("a");
		}
	}
}