There is a new .NET Micro Framework board that is 100% open source now. The netduino from Secret Labs can leverage the arduino shields! Their projects page, http://www.netduino.com/projects/, has a few projects like how to get a blinking LED to work and read data from a color sensor to determine was color is in front of it.
Netduino combines 20 GPIOs with SPI, I2C, 2 UARTs (1 RTS/CTS), 4 PWM and 6 ADC channels. Best of all with NETMF 4.1, you can use Visual Studio 2010!