The idea itself is not new. GHI Electronics have a product called FEZ domino which is quite similar. What sets Netduino apart is the open-source firmware and the cheaper, $35 price tag.
I’m placing an order for one. I’ll post my review after I receive the board.
Update (21 Oct 2010):
I’ve received the board and have posted my initial impression.
I just received my Seeed Studio DSO nano oscilloscope.
I had difficulty justifying the purchase of this oscilloscope, as I had doubts on its usefulness and performance. However, the unit has such cool form factor that I just had to have it. The oscilloscope is modestly priced at USD $89. I justified buying it because, if for some reason the oscilloscope is not usable, then I still can use it as an ARM STM32 development board (Hey, this is the same CPU that powers Hexi’s locomotion engine).
Sometime buttons need to be debounced in software. This article has good information on how to do it. The simplest method of debouncing is by sampling the switch every 50 milliseconds.